bdp24 wrote: "I understand the subs in a swarm design all reproduce a combined left plus right (monaural) signal, even on material containing stereo bass (rare, but not unheard of). Is that correct?"
That’s normally true of the system I make. The amplifier I use is the Dayton Audio SA-1000, part number 300-811 at Parts Express, normally one to drive all four subs. So you can get that amp and four passive subs connected in series-parallel and that’s pretty much what I do.
"Is there any out-of-phase (left minus right), very low frequency, info lost when doing so?"
Could be.
Some of my customers opt for using two amplifiers, instead of a single amp to drive all four subs. This way they can send the left channel signal to the left-side-of-the-room subs, and the right channel signal to the right-side-of-the-room subs.
I think most of my customers who are using two amps use the variable phase controls to set the subs roughly 90 degrees apart, in "phase quadrature". This synthesizes partially out-of-phase conditions at the left and right side, increasing the sense of envelopment or immersion. Credit to David Griesinger for this technique. Then if you have a recording that you know has true stereo bass information, you’re just a phase-knob-twiddle away from hearing it fully.
Duke