That was a bit elusive, yes. I tried to touch upon that using a sub setup the way I outlined as being my preference is a matter of having a different "design" goal - i.e.: high-passing the mains fairly high to a pair of subs, (the need for) symmetrical subs placement, high sensitivity, pro drivers, very large overall size and usually not direct radiation - all in the service of loads of headroom in the lower frequencies (that translates into easy of reproduction and very low distortion) and optimum integration.
@phusis
If you're crossing higher than about 80-100 Hz, won't you have to place the subs very near the rest of the speaker system?
I get the desire for a high efficiency sub- that prevents thermal compression from being as profound. FWIW I only have 2 of Duke's Swarm subs, as my CAL T-3s take care of everything down to 20Hz in the front of the room. Even at very high volumes, I'v found that the Swarm's drivers aren't moving much. I imagine that could be very different in a bigger room. They are really only there to break up the standing wave though, and apparently don't have to move as much as the front woofers (dual 15" in each cabinet) to do so. So I'm not concerned about thermal compression in this case.