Very good questions, John!
The subs all receive the exact same signal, as they wired in series-parallel and are all driven by a single output channel from a shelf-mount plate amp.
I investigated other configurations and concluded this was the most cost-effective. The amp puts about about 1 kilowatt, so each sub gets about 250 watts. The variable low-pass filter is 4th order, so it's steep enough that you can get away with positioning subs well away from the main speakers without too much lower midrange energy leaking through and betraying their presence. The amp includes a single band of parametric EQ, so that if there is a residual frequency response anomaly in the bass region it can be addressed (so far, to the best of my knowledge, none of my customers have used it).
For an all-out system, there is an argument for using left and right channel amps. Not because frequencies below 80 Hz are in stereo, but because it would be possible to operate one channel 90 degrees out-of-phase relative to the other channel, which according to one researcher results in an improved sense of "envelopment". The amp has a continuously-variable 0-to-180 degree phase control, so you'd set one amp "normal" and then dial in 90 degrees of phase shift on the other one. I haven't tried this.
The enclosures are tuned to have a roughly room-gain-complimentary 3 dB per octave rolloff across the bass region. In my opinion this avoids bass heaviness that you can get if the subs are "flat" anechoic, as boundary reinforcement tends to gradually boost the bass depending on the specific room acoustic situation, with 3 dB per octave being of course an approximation. In situations where room gain is more than 3 dB per octave, you can wire one of the three subs in reverse polarity relative to the others. It would tend to add in semi-random phase in the upper bass region (assuming the subs are scattered), but would tend to cancel the other subs in the very low bass region, thus somewhat counter-acting the effects of strong room gain. Or, you could just use the parametric EQ function. Lots of options.
