Comparison of various subwoofers for 2 channel audio

@drbond  Yes please, you should. That will be a 60 dB/oct filter which is very slow. You might want to try setting the HPF to 100 Hz and the sub LPF at 80 Hz. I spent years with Acoustats, RH Labs Subwoofers and the Dahlquist LP1. To set the HPF you were given an assortment of caps and the formula for determining the 3 dB down point given your amp's input impedance. I tried every combination you can think of. It was a very euphonic system and anything but accurate. Getting subwoofers to disappear running up to 80 or 100 Hz is not easy. Most have enough coloration to stick out like a sore thumb especially if you have very uncolored loudspeakers. 

@drbond Yes is was. I had to add another set of input terminals too the back plate. Dr West was kind enough to send me an identical set. The holes had to be right on. All crossover components were removed leaving the two transformers, the bias supply and the output terminal block. Primary resistors were added to the high frequency transformers.

Initial results sucked. The bass transformer is way less efficient than the treble transformer and the Atma-Spheres have less gain than your typical SS amp including the Bricasti M25 I am using to drive the high frequency transformer.

Then because of the dipole nature of the Sound Labs the DEQX was having a terrible time getting measurements, comb filtering like crazy. I must have tried 100 different set ups. I realized that the only way I was going to get this to work was by blocking the rear wave almost entirely and adding about 12 dB of gain to the Atma-Spheres. I built two of Dr West's SALLIEs (Sound Attenuation for Low Levels of Interference Effects) except mine are 24" wide instead of 12" and 7 feet tall. I will have a picture of them on my virtual system page shortly. I purchased a unit that converts single ended signals to balanced signals based on a older op amp, set for unity gain. I substituted a Burr Brown op amp and raised the feedback resistor by a factor of 4 which will give me 12 dB of gain. I have a set of Burson op amps coming. These are the highest quality available but very expensive. I wanted to make sure this was going to work before spending that kind of money. 

The bass control on your sound labs cuts or increases the volume of the bass transformer and functions from 500 Hz down. By lowering the bass you are taking the punch out of the music. Unless you are going to get a two way crossover I firmly believe you are better off without subwoofers. Adding more of them is simply going to increase coloration in the low end. The MAJOR benefit with your speakers is lowering distortion and increasing head room by removing 100 hz down from them. This is more important than the added bass. If you are every up in the Boston area I will prove it to you. 

You are stuck in old school thinking, like late 70's old school thinking. You have a great system, but as it is it is extremely inaccurate. The amplitude curves for the individual channels are way different, up to 10 dB in places and anything but reasonably flat, think roller coaster. Get a USB  measurement microphone and computer program and measure it. I promise it will set your hair on fire. The benefits of digital signal processing far outweigh any detrimental effects which with the best equipment are completely inaudible. 

@drbond Your speakers are 7 feet tall. How high is the ceiling? If it is over 7' 2" tall the behavior of your speakers drop to point source at about 130 Hz. If you are crossing below 130 Hz it is probably more accurate to stick with the two subs you have, cross at 100 Hz and turn up the volume on the subs. They should be about + 10 dB. What are you using for a crossover? 

I have succeeded in bi amping the Sound Labs. Roger helped with the conversion. There are no passive crossover parts in the system. It was not easy going, but the end result is pretty amazing.