Who are some of elite sub woofers?

Awoof, one possibility since you already have a sub, is to add a few more.  In a distributed multi-sub setup, the subs need not be identical.   It is perfectly fine to have one or two that go significantly deeper than the others.  

If you go this route, let me suggest that any subs which will be closer to the listener than the main speakers have a steep lowpass filter, something like 24 dB per octave, to roll off their top ends.  You don't want them passing audible upper bass/lower midrange energy and giving away their locations. 

Also, a phase knob or switch can be nice to have.   I usually end up reversing the polarity of one of the four subs in a Swarm setup, as I find that this usually helps to further smooth things out.

Duke

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