InnerSound Eros Mk-II Speaker System

InnerSound Eros Mk-II Speaker System


Frank Alles

2 August 2002


Speaker Dimensions: 
15″ W × 18″D × 68″ H (381 × 450 ×1700mm)
Speaker Net Weight: 80 lbs. (33.5 Kg)
Shipping Weight: 100 lbs. (44.7 Kg)
ESL Panel Size: 13 1/2″ × 42″
ESL Power Handling: “Unlimited for any amplifier intended for domestic use”
Bass Power Handling: 600 watts
Bass Driver: 10″
Bass Design: Transmission Line
Sensitivity: 96dB/2.83 volts/meter
Frequency Response: 24 Hz to 27 kHz +/- 2 dB

Crossover-Bass Amplifier
Amplifier Dimensions: 17″ wide× 5.5″ tall × 14″ deep (43 w ×14 t × 36 d cm)
Amplifier Net Weight: 42 lbs. (19.0 Kg)
Amplifier Power: 600 watts RMS per channel @ 4 ohms
Unbalanced RCA and Balanced XLR input and output connections
Price: $5,995.00 USD + S/H. 

InnerSound, Inc.
1700 Hwy 16
Whitesburg, GA 30185
Tel: (770) 838-1400
Fax: (770) 838-0111

Tech Support:

The new Eros Mk-II has recently superseded the original, highly acclaimed Eros speaker system. The revisions are extensive, involving both the speakers and the electronics, and will be detailed in this article.

InnerSound is one of the very few high-end companies that, should you choose to use their gear exclusively, you are almost guaranteed to end up with a great sounding system. This applies not only to their speaker systems, but to their phono preamp, line stage preamp, power amplifiers and their excellent interconnects and speaker cables. To me this is no coincidence and no small accomplishment. It is the result of a very talented designer putting the same 110% effort into each and every product his company produces. This effectively takes the guesswork out of system matching. Just insert any competent digital or analog source into an InnerSound system, and with a little careful attention to proper set-up, short of having really horrible room acoustics, you can’t miss. Not to be overlooked is the fact that InnerSound’s chief designer, Roger Sanders, is very knowledgeable and so willing to share his knowledge, often going those extra 9 yards to satisfy his customers’ individual concerns.


The Eros Mk-II system consists of a pair of hybrid speakers. Each speaker uses a 10″ woofer in a transmission line enclosure supporting a 42″ × 13.5″ ESL panel. The system that I tested is the bi-amplified version that includes an integrated, active crossover and stereo bass amplifier on a single chassis. There is a passive version of the Eros that does not include the active electronics that sells for $3995.00 USD.

Although the Eros speakers are physically large, they weigh only 80 pounds each and are easy for a single strong person to set up and maneuver about the listening room. For me, this is a very real consideration, especially when some competitors’ speakers tip the scales at over 200 lbs. per speaker.

This new crossover/amp has been totally redesigned and is now made in the USA. The internal amplifier is based on InnerSound’s very successful ESL amplifier, which it matches aesthetically, and is offered with either a black or silver faceplate. It provides 600 Watts per channel to drive each 10″ woofer, as opposed to the original’s 200 Wpc rating. It has separate low bass and upper-bass/lower midrange controls, so it is possible to achieve a very good balance in the majority of listening rooms. The crossover frequency has been lowered in the Mk-II version from 450Hz to 360Hz and the slopes are steep at 24dB per octave for both high- and low-pass sections. A sophisticated, electronic, master level control has been added, which provides one hundred, 1-dB steps, and channel matching to within 0.1 dB. The unit has a blue, digital readout that shows the level of each function and is similar to the display found on the InnerSound’s separate preamplifier.

The included Home Theater Master SL-9000 learning remote makes system operation almost too easy. One can easily adjust the volume and set bass and midrange balance on the fly from the listening seat for any recording played. The SL-9000 can also be programmed to control almost any other infrared remote controlled components that you may own. Other improvements include thicker baffles in the transmission line woofer enclosure, improved, more robust ESL panels, and a modular plug-in crossover board in the crossover/amp. 

The Mk-II crossover/amp includes both RCA-type, single-ended inputs and outputs, plus XLR balanced inputs and outputs. There are outputs for the midrange amplifier as well as separate bass outputs. I can’t imagine anyone needing a better or more powerful bass amp than the one that InnerSound includes, but I used the bass outputs on my unit to feed a Paradigm Servo-15 subwoofer for a little more ultra-low bass reinforcement.

I have owned both the original Eros, and now, this latest Mk-II version. In my view, the Mk-II’s redesigned crossover/bass amp is a huge asset to the system. It provides performance gains by allowing independent adjustments of the deep bass, and the upper bass to midrange balance. The electronic master level control is exceedingly transparent (like it’s not in the signal path) and finally provided me with an excellent way of gaining remote control without compromising my system’s performance. 

The latest Mk-II crossover/amplifier handles up to 10.5 volts of input signal before it will overload. This will allow one to use almost any source without fear of overloading the input stage. This is an important point, especially if you plan to use an active preamplifier ahead of the crossover/amp (since some preamps boost gain by 20 dB or more). In some instances where a high-output source component is feeding a high-gain preamp, an output voltage greater than 10.5 volts could possibly result. But, in such a rare case, one can simply turn down the volume of the preamp feeding the crossover/amp until no overload exists. Certainly there is plenty of gain in this recent iteration to drive any normal audiophile-approved power amp well beyond clipping without overloading its input stage. With the volume pot of my AHT tube line stage set to a normal level I did not experience any input-clipping with any combination of source components and power amplifiers that I tried.

Even the universal learning remote is cool-so far I have programmed it to operate my Parasound CD transport and 4 other components (receiver, VCR, DVD & TV) in my Home Theater system. Not only is the sound quality of the Eros Mk-II system among the best available, its user-friendly features are very easy to appreciate.

Perhaps best of all, those of you who use a solitary sound source will not even need a preamp-simply run your source component’s stereo outputs straight into the crossover/amp’s inputs, set the volume level and bass parameters, then sit back and enjoy.

Since InnerSound provides an excellent bass amplifier, the user need only supply one amplifier to drive the ESL panels, albeit, one that is robust enough to deliver adequate current into 2-ohm (at high frequencies), capacitive loads. The three amps that I used for my evaluation, the InnerSound ESL amp, and both the Monarchy Audio SE 100 and SE-160 monoblock amplifiers, all produced great results when driving the original Eros and performed similarly with the Mk-II version. InnerSound plans to introduce a tube amplifier in the near future and I can’t wait to try that baby out on the Eros Mk-II when it becomes available.

Listening: The Orgasmic Experience

During my evaluation of the InnerSound Eros Mk-II, I heard so much more detail from almost every cut in Fionna Apple’s When the PawnCD [Clean Slate/Epic EK 69195]. Although I have played this recording countless times, it seemed as if I was hearing it for the first time. A drum/percussion passage toward the end of her song “Limp” amazed me by how dynamic and true to life it sounded (very high slam factor). Later I played the soundtrack from O Brother, Where Art Thou? [Mercury P2 70069], and almost jumped out of my skin at the start of “Po Lazarus” because the sound of the prisoners cracking rocks in the opening was just THAT STARTLING!

Playing through the Squirrel Nut Zippers’ album, Hot [Mammoth 354 980 0137-2], turned out to be a very pleasant experience. The brass instruments sounded quite real and full-bodied and the bass seemed a bit better defined and displayed more sock than I had witnessed with the original Eros. I really dig that retro-swing music, and the performance of the Eros Mk-II was remarkably similar to what I hear live.

Suffice it to say that all types of acoustic instruments, i.e. piano, brass, woodwinds, and especially strings of all genres, sounded rich, vibrant and authentic through the Eros Mk-II-with all their nuances and fine details preserved intact. Both male and female vocals are particularly involving, and some performances, can be riveting when “sung” through the new Mk-II.

Imaging is another strong suite of the Eros Mk-II. The soundstage is very expansive laterally and has a very good sense of depth and layering. Instruments are precisely located and do not wander no matter how complex the mix. Listening to Fleetwood Mac’s “Tusk,” from The Dance [Reprise 9 46702-2], I and others were amazed when the University of Southern California’s marching band armed their battalion for the cause and joined in the fray. Complex? You bet! Yet the Eros Mk-II held the line, keeping the various instrumentalists in their proper places. It was colossal; it was immense; it was horns to the walls-but it was child’s play for the mighty Eros Mk-II. T-U-S-K! On the next cut, “Don’t Stop,” the percussion section is highlighted near the end of the song and sounds quite dynamic and realistic. In that instance I concluded that the song was well titled, because I really didn’t want it to end.

Regarding the bass performance, this is an area where the Mk-II version outshines the original. The low bass can be boosted quite a bit with the electronic bass control, and even when using high levels of boost the woofers can be driven fairly hard without losing their composure. The power and control of the integral bass amp is substantial.

Martin Logan – NOT!

How does it compare to the Martin Logan hybrids, you ask? Certainly there are differences. The most apparent sonic differences between the flat ESL panel of the Eros and the curved ESL panels found in Martin Logan models are as follows: The Eros flat panel design causes noticeable beaming of the high frequencies starting about 1000 Hz on up. Because of the beaming the ESL panels must be toed in more than other speakers-aimed directly at the listener for optimal treble performance. This results in considerably less high frequency room reflections. The beaming also reduces high frequency crosstalk between channels, resulting in a semi-binaural effect, which bears certain similarities to the type of presentation provided by headphones. The Eros provides an expansive soundstage, with clearer, more distinct instrumental images, albeit, in a somewhat narrow sweet spot.

Indeed, the high degree of clarity and precise focus that the Eros Mk-II provides must be heard in order to be fully appreciated and comprehended. In this particular regard, the Eros surpasses virtually all other speakers (dynamic, ribbon, or ESL) that I’ve encountered. The flat panel design also boosts efficiency and improves system dynamics. By contrast, the curved panel design of the Martin Logan results in broader dispersion of the high frequencies and a wider sweet spot-but also a more diffuse, less focused sound. Personally, I always sit in the sweet spot when I listen anyway, so for me, the better clarity and incisiveness of the Eros make it my choice whether listening critically or solely for pleasure.

Additionally, InnerSound has developed specialized diaphragm coatings that make their electrostatic panels essentially bulletproof. InnerSound speakers are widely used even in extremely humid areas like Japan, Thailand, Costa Rica, and the Philippines without failure. Roger Sanders claims his electrostatic panels are arc-proof and cannot be damaged by over-driving, and that they neither need nor use any protective circuitry. In fact, InnerSound is so confident in the ruggedness of its ESL panels that they now come with a lifetime guarantee. 

Likewise, Sanders is very proud of the transmission line woofer loading used in the Eros, claiming: “It has much better damping and transient response than the closed-box woofer used by Martin Logan. The TL (transmission line) virtually eliminates overshoot and ringing with the result that the bass response of the Eros is so clean and fast that it blends seamlessly with the electrostatic panel. No other electrostatic manufacturer, ML included, has achieved such perfect integration between such fundamentally different driver types.” 

Certainly a designer’s opinion of his own work will tend toward the optimistic; and while perfect driver integration is the goal, it may be more of a dream than a physical reality. That said, I agree that the Eros achieves demonstrably better driver integration than the Martin Logan and other hybrid designs that I have encountered. However, as seamless as the blending of the Eros’ drivers may be, it does not quite equal the coherency of an electrostatic loudspeaker that covers the bass/midrange region without a crossover, such as the large Sound Lab designs. Then again, the Eros appears to better the Sound Lab models in terms of bass dynamics and extension. Plus, the Eros’ less imposing physical presence allows it to be used in rooms of more modest dimensions. 

[Note: Upon seeing my comments regarding the bass/midrange driver integration, Sanders e-mailed me a very enlightening and informative commentary comparing the bass performance of full range ESLs to the Eros’ hybrid design. His thoughts follow this review.]


As with all electromechanical transducers there are always shortcomings. As excellent as the Eros Mk-II system is, some aspects of its sonic performance leave room for improvement. My first concern is its narrow sweet spot, which is perhaps 12″ to 18″ wide at best. Two people listening side by side will find it difficult for both to occupy this prime real estate and will more than likely find that listening seated one behind the other is the best arrangement for sonic concerns. This is unlikely to be improved much, because it is a function of the flat-panel design, which is responsible for the precision focus that I noted above. It’s sort of a catch-22.

My other concerns lie in the speaker’s bass reproduction. Although the bass performance is quite adroit, I feel that the bass enclosure could be a bit more inert. Also, the “midrange” control on the Mk-II version appears to affect the upper bass to a large degree. I had some trouble setting the midrange control without making the upper bass too prominent or the midrange too lean. Of course this critical balance is somewhat dependent on room acoustics and speaker positioning. I may yet find a better position for the speakers that will minimize this tendency.

Roger Sanders suggested cranking the low bass up while decreasing the midrange level and that did help, to a degree. Still I thought that the transition from upper bass to lower midrange could be improved. Curiously, the bass/midrange balance varied somewhat according to which (midrange) amplifier was used, with the Monarchy SE-160s exhibiting more apparent energy in the upper bass region than the other amps I tried.

I consider the above concerns to be relatively minor in the face of the system’s overall high level of sonic reproduction.

Summing Up

After many hours of listening to the Eros Mk-II speaker system, it is my opinion that it rivals today’s most highly regarded systems in terms of its overall sonic performance. Its asking price is a bargain and the recently revamped crossover/amp is the icing on this very tasty cake.

The Eros’ reproduction of acoustic instruments is exceedingly convincing and its portrayal of the human voice is simply captivating. While the amount of musical nuance and fine detail it unravels is sure to impress, the Mk-II’s ultra-clear, incisive presentation is in a class by itself. The Eros Mk-II is a very capable, musical, and articulate system from top to bottom.

It is my hope that more audiophiles will discover the benefits of InnerSound ownership and Roger Sanders’ fantastic customer support. Toward that end, InnerSound offers a risk-free, 30-day, in-home trial where the customer can return the product for a full refund for any reason at all. I’ll be keeping my Eros, thank you very much!

Roger Sanders comments:

Dear Frank:

You may be interested in knowing that for most of my life (well, the last 35 years of it anyway) I have used full-range, crossoverless ESLs in my personal music system. So, I have a lot of experience with electrostatic bass. 

Like most audiophiles, I have always expected that a full-range, crossoverless ESL was the best possible speaker, and that the bass from an ESL should be virtually flawless (if one overlooks its obviously severe deficiencies in output and depth). I’m referring to how “clean” and “tight” it is — in other words, the massless nature of an ESL should allow it to produce perfect bass transient response. 

But this is not at all true! While an ESL is essentially massless, the air it drives is not. Therefore, an electrostatic diaphragm will have a very large fundamental resonance (16 dB typically) where the air mass “seen” by the speaker resonates with the “spring rate” (the tension) of the diaphragm. In large ESLs, this resonance lies somewhere between 50 Hz and 100 Hz. 

As with any resonance, there is a tremendous amount of overshoot and ringing when reproducing frequencies near resonance. Therefore, full-range ESLs actually have rather flabby and poorly controlled bass in their deeper regions. 

They do have beautifully controlled upper bass and that is where they get their reputation for good bass. And overall, their poor bass transient response is still better than most dynamic systems. So on the whole, it is understandable that many audiophiles consider the full-range ESL to be the best. But if evaluated objectively, with an open mind, their bass is really pretty bad. 

The fact is that a properly designed and executed TL design will have NO resonances at all! Its rise time, fall time and transient response will be far superior to a full-range electrostat. And of course, it will actually be able to reproduce deep bass and have high output. A full-range ESL is really no match for a TL in any way you care to compare them.

Of course, when using a TL, you have the issue of crossover networks. But, it is here, in the upper-bass/lower midrange, where the full-range ESL slightly outperforms a good TL, even when electronic crossovers are used. The difference isn’t great, but it is there. And there is really nothing good to be said about crossovers and there is simply no way that a crossover can outperform a speaker that doesn’t have one. So the full-range, crossoverless ESL has a slight edge over a hybrid in this one area only. On balance, the ESL/TL hybrid outperforms a full-range ESL so dramatically in the bass that comparisons are silly — and with a good electronic crossover, the hybrid is almost as good in the lower midrange. I guarantee that you won’t be anywhere near as happy with a full-range ESL as you are with our Eros.

Great listening,

Roger Sanders

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