Subwoofers and electrostatic speakers

There have been a few threads about this in the past, but I thought I would share my experience in this thread:

I have had electrostatic speakers (ESL) for the past 20 years. I initially had QUAD ESL’s, but have since upgraded to Sound Lab ESL (Majestic 745). I have always appreciated the detail and clarity that ESL’s brought to music reproduction. I have only listened to classical and acoustic jazz music so, the well known fact that ESL’s lack the bass slam, didn’t deter me from the product.

However, in the past few years, I have pursued more reality to the musical presentation, and upgraded my entire 2 channel stereo system, and the results have been very rewarding along the way. I have been quite pleased with the musical reproduction of my current system, but when playing larger orchestral works, it’s easy to note that the tympani sound distant, and the double bass almost imperceptible. However, with smaller chamber and solo instrument pieces, the reproduction was near perfect. Because of the former deficiency, I decided to try integrating subwoofers into my system.

I communicated with several members of this forum, and I considered their recommendations. Based on my current experience, I think the recommendation by @mijostyn to purchase Martin Logan Balanced Force 212 subwoofers was a great piece of advice. They were manufactured to integrate with ESL’s, are not ported, and are designed to prevent the distortion that other subwoofers can cause with the bass sounding too slow and "boomy" when integrating with ESL’s.

I was told that getting the subwoofers would be the easiest part, but that integrating them with the ESL’s with proper high-pass filters (HPF) and low-pass filters (LPF) would be more challenging. There were all sorts of recommendations about HPF’s, including passive vs active vs analog vs digital. The LPF was easier, as it’s integrated into the subwoofer itself.

It seemed that integrating a HPF would be most challenging, and all methodologies would change the sound quality (SQ), and since my system was very transparent, detailed, and musical, I didn’t want to interfere with the superb SQ, whether that be integrating a capacitor into the interconnect feeding my amplifiers, or changing a pre-amplifier to some sort of DEQ device with unlimited fidding with orders of filters, etc. 

Ultimately, I decided not to even bother with using a HPF, and, as you can determine from my description below, I am very content with the SQ and musical presentation without any HPF. 

I decided that I would start simple, and I just connected the subwoofers to my preamplifier via interconnects, and played with the LPF, volume, phase, and position of the subwoofers. I decided on a rather simple placement of the subwoofers, just to the inside of the larger ESL’s, and had to adjust their positions slightly, but not markedly, as it turned out that the best stereo SQ was achieved by slightly staggering the subwoofers, one about 18 inches closer to the listener than the other, but both just to the medial (inside) aspect of the ESL’s.

I toyed with the phase and volume, but it turned out that simple was the answer: 0 phase shift, and middle volume worked best.

The LPF wasn’t too tricky either: with the Sound Lab ESL, I started with the LPF at 30 Hz, and just moved it up until I heard too much, and then backed it off a bit. It turns out that 50-60 Hz for the LPF did sound best, and the subwoofers must have a high order LPF, because I hear nothing discernible above those frequencies, even with the ESL’s off, playing music only through the subs.

These subwoofers don’t interfere with any of the SQ from what I can hear playing digital sources. If I play a solo piano from an analog source, I do hear a slight change to some imaging with only a very few notes on the piano, but I don’t hear this with any other instrument, and there’s definitely no perceptible detraction in imaging with larger compositions, involving more instruments.

Overall, adding the subwoofers to my system was a significant improvement in the overall musical presentation. I was surprised to hear that even in solo instrumental pieces, that the subwoofers added to the overall presentation. In larger, orchestral pieces, the subwoofers added markedly to the overall presentation, especially with the tympani, double bass, or continuo. Even in more modestly sized compositions, the subwoofers added to the musical experience. Overall, I would say that the subwoofers made the SQ sound more "full" or "robust", while not detracting from the clarity. Everything sounds like a more complete performance now, and brings me closer to the recording experience, insofar as the slight movement of the ground and air pressure around me causes me to feel "closer" to the performers, whereas previously, with only the ESL’s the SQ was slightly (although imperceptibly so, if you don’t compare the SQ with subwoofers) on the sterile side.

Thanks to many on this forum for their recommendations and suggestions in my musical journey!


Subwoofers are usually able to complement any speaker.  The trick is always getting it set up properly with the room and the speakers.  It's like finding a partner your children and your parents like. 

I would recommend you get the Martin Logan Unison preamp/streamer. It has a sub out port AND ARC room correction. I use one and it is amazing, even more amazing at the sale price of $199. ARC will integrate your speakers and your subs and the preamp has that ML signature sound, you will love it:

You have a spectacular system.

I’ve found there are a number of LP’s that simply sound fine without my subs help. The digital releases of those same recordings sound better with the subs on. Another confirmation that high pass filtering is not preferred in the context of my system.

Despite my subwoofers uniquely thorough method of Auto EQ the first step in the manual requires preforming what’s now referred to as the subwoofer crawl. The crawl takes the trial and error mystery out of perfect subwoofer room integration at the listening position.

Once your rooms mode locations are identified those positions will work with any subwoofer/s and may possibly be a weakness in your current subwoofer positioning.

@drbond I suggest you take the next step with subwoofer integration with mains and use  a miniDSP UMIK-2 microphone, software REW and MSO, and a miniDSP 2x4 HD to achieve equalized and more uniform around the room bass response.  The mains signal can remain analog and I suggest a passive first order HP filter implemented in an interconnect extension.

The starting URL for information is

I have two QUAD ESL 63 (USA monitor version) integrated with three JL Audio E112 subs but in theory the subs could be mismatched and anything suitable. The mains are on an 80Hz passive HPF and the subs have a LPF of 70 Hz, Butterworth 24 db, inplemented in the 2x4 HD.


Yes, I think every system could benefit, at least in part, to the addition of subwoofers.

I have to stick with my pre-amplifier, which is Ypsilon. The delicacy, detail, and musicality of this pre-amplifier is as good as it gets, although there are probably others that are just as nice. This precludes a digital DEQ type pre-amplifier.


Yes, I could try numerous locations for the subwoofers, but at 140 lb each, I really don‘t want to move them much. My room is also very irregular, with an open side wall into the house, which apparently makes it easier to find a decent placement for the subs without producing sound interference patterns.

I also find that there’s more resonance (or something) in analog sources, which apparently doesn’t need as much of the extra bass that subs can offer; however with larger, more complex compositions, I find that the subs do augment the music well.


Yes, if I were to proceed more scientifically, I would get the REW and microphone, but, as I mentioned my room is very irregular and I couldn’t really insert a proper dimension for the room.

I may try a HPF via a capacitor in an interconnect extension in the future, but I’m told that this will affect the SQ, which is currently sublime in my system, which is why I am reluctant to do so.  I also listen to enough solo instrumental music without the subwoofers that the HPF would have to be removed during such episodes, which would just be another detail to switch back and forth.

OK, then you might want to try a sub equalizer from DSPeaker, the 8033. 

@drbond, you should try an HPF and determine if it is acceptable. The key with subwoofer optimization is MSO.  I works regardless of the room parameters.

@drbond    Yes, I could try numerous locations for the subwoofers, but at 140 lb each, I really don‘t want to move them much. My room is also very irregular, with an open side wall into the house, which apparently makes it easier to find a decent placement for the subs without producing sound interference patterns.

Numerous trial and error sub positioning is exactly what the crawl eliminates. Regardless of a rooms irregularities, to excite the rooms standing wave modes temporarily place a sub at the listing position and play the low frequency test tones. Walking around the room you'll notice the loud standing wave mode locations. Most any -3dB rated subwoofer will room integrate perfectly in relation to the listening position from any of those identified locations.  

The crawl is a procedure that need only be done once. These locations can reduce the degree of frequency processing needed while optimizing the sense of timing or room lock.


I found wrestling heavy subwoofers absolutly dangerous! My subs reside on birch plywood dollies using 5" 'extra soft' rubber casters topped with 4" of firm latex foam. After a recalibration I found the decoupling increased definition noticeably while reducing the low frequency transmission throughout the dwellings structure. 


@drbond I have a similar set-up (1 ML BF210 and ML CLXs). ML provides custom crossover firmware(s) for all/most their models on their website, that can be uploaded into your BF212 with a USB stick. I run my CLXs direct from the line stage. You might want to see if any of the ML models are close to the frequency response of your Sound Labs and try it.


What I would do for sure is to get a Perfect Bass Kit (PBK) from ML and create a custom room response filter and upload it to the ML212’s. This will give you much more musical bass, as the loudness up and down the scale will be much more even, and has a bigger impact than the custom crossover. I suspect this is potentially even more important with your unusual floor plan. I will note the I could not get the newest version of the ARC Genesis software to work on my MBP, so I had t borrow a Win11 laptop from work to measure the bass frequency curve and upload the inverse curve to the BL210. Note I only have one. For two you would correct each one individually, and the compensate with a combination of the overall level of the two. I would talk with ML and Anthem about that for advice.

@delm @docknow @m-db

Yes, the Martin Logan version of MSO is the PBK. The problem is that with current placement the subwoofers sound fantastic, without all the complicated DEQ, PBK, HPF, etc.

Before the purchase, I was told that the subwoofers and electrostatic speakers are poor matches, and I was prepared to follow through with complex HPF, etc, if necessary, but either I’m just lucky with my placement, and set up, or the Martin Logan Balanced Force subwoofers just match perfectly with my Sound Lab electrostatic speakers, with simply placing the subs in a standard stereo postiion. Consequently, I’m reluctant to start with endless fiddling when it’s perfect now, with only a slight stagger to the subs. (I did note that if the subs were placed symmetrically, that there was a band of low frequnency sound that got cancelled out between the two subs.)

However, since the ML PBK seems quite simple to use, I might try that in a couple of months, but I suppose I’ll have to investigate how to use the PBK with two subs, and if two subs makes the PBK obsolete, or less effective.

I thought my one 210 sounded great, so I procrastinated running PBK for a while, until the results of PBK the first time.  Now I do it as soon as I can when setting up the system in a new location (3 since that first time)

I have spoken with ML about PBK with two subs, as I am on teh fence about getting a 2nd 210.  PBk works with two subs.


Yes, I read that the Martin Logan PBK works with up to four subwoofers; however, it analyzes each one independently.  I would think a more effective way would be to measure all the subwoofers together, since there may be interference between two or more subwoofers.  Regardless, I think I’ll buy the PBK kit this weekend, and run it in a week or two, and try to discern what difference there is.  As I understand it, if I don’t like the results, I can always turn the PBK programming off, and revert to the present state.




So, I got the Martin Logan Perfect Bass Kit (PBK), which is room EQ software that can either be automated or manually set up using a microphone measuring from 5 different points in the room.  
Firstly, l have really been appreciating the marked improvement in texture that the subwoofers give to all musical instruments with just the standard operation using the LPF set at 55 Hz in conjunction with my Sound Lab electrostatic speakers.  Regardless, I followed through with proper room EQ set up using the PBK, and at first I was rather pleased:  the bass sound did sound more refined, slightly more precise, and not quite as thick at the lowest register.  
Without any EQ, my room has an increased signal from 20-40 Hz, and then dramatically drops between 40-50Hz, before slightly rising back up to ideal flat signal after 60 Hz or so.  After the PBK, the Room EQ flattened out the signal rather markedly, to make it appear almost ideal.  
When I first listen with the Room EQ, as I mentioned, the bass sounds more refined, and perhaps slightly more precise, but it does seem to detract from some of that more visceral component of the lower frequencies.  If I only listen for an hour or less, then I am content with the Room EQ; however, I noticed that if I have a longer listening session for a few hours or more, that I get rather significant ear (listening) fatigue, which has never before been a problem with my system.  So I turn the Room EQ off, and listen to the standar subwoofers without any EQ, and the fatigue immediately disappears…so I’ve reverted to listening to the subwoofers without any EQ, and I think that it sounds superior, although it measures worse!

So, that somewhat supports my opinion that audio measurements are an idealized representation of what some people think music should sound like but they are mostly wrong…

Does PBK provide adjustment of anything other than frequency response?

Manual adjustments? Multiple memory presets?


Yes, you can manually adjust the subwoofer via the computer interface with the PBK.  I don't know about multiple memory pre-sets.  I think you can only save one Room EQ setting, and you have to delete that  one before you can install another program. 


While adjusting frequency within a number of bands is an important parameter to correct does PBK offer any other manually adjustable parameters such as Xover and subsonic slopes, parametric filter frequency, contour levels and Q bandwidth? 

The ability to manually adjust parameters can be quite critical toward your personal  "idealized representation of what some people think music should sound like"  Very well said and an issue with much of consumer subwoofer DSP. 

Reading your eloquent description I can't help but feel you are so close.


The PBK uses ARC Genesis software, which is Anthem Room Correction Genesis.  The description seems to indicate that the manual adjustments are nearly unlimited, but I don't know exactly how many parameters could be adjusted, and if I tried, I think I would probably just make things sound worse, after days of infinite adjustments and tweaks.  

Here's what their website says:

"Designed for both consumers and professionals, ARC Genesis (available for Mac and Windows) offers Auto and Professional modes. The streamlined user interface of ARC Genesis makes it easy to perform corrections that provide excellent sound in any room and system configuration—ranging from simple 2-channel stereos to elaborate 9.4.6-channel home theaters.

Ideal for most consumers, ARC Genesis’ Auto mode asks a few simple questions about a system, runs a full set of measurements, automatically calculates the correction and bass management settings, and uploads them to the ARC-compatible device. Ideal for professionals and advanced audio enthusiasts, Professional mode offers the full gamut of controls and allows a granular level of customization before uploading corrections and bass management settings."

Your next step, ie., after you understand the intricacies of subwoofers would be to move to the "fast sub" options

GR Research - Sealed, open baffle, servo controlled

Rythmik - Sealed, servo controlled

These are the only subs (currently available) that work best with magnepans, ML electrostats, etc...


You don’t think that the Martin Logan subs are designed to work with the Martin Logan electrostatic speakers?

@drbond , Rarely does a speaker manufacturer produce the caliber of subs produced by niche sub manufacturers. Even if they were able to, they wouldn't be able to get around patents from the likes of Rythmik.