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!