@atmasphere --
"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?"
Yes indeed, no way around that. You can see it in my system's picture where the two tapped horns are flanking more or less the mains being positioned just behind them in their respective corner. Corner loading, as I'm sure you know, is a double edged sword; there's boundary gain but also a typical addition of peaks and nulls in the frequency respons. In my case it's a fairly decent outcome with limited absorbtion implemented, the biggest audible issue being a slight suck-out in the midbass region and a (more manageable) peak just below around 30Hz. All in all it makes for a smooth integration with the mains, and I've never seen the 15" B&C drivers in the TH subs move (or rather vibrate) more than a single mm or two at most - even at volumes where the air shakes violently for a very visceral and immersive experience.
"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."
It's a smart way to make a DBA, using the mains full-range and thus adding two bass sources in addition to the subs. Where are two Swarm subs positioned in relation to the CAL T-3's? It would seem the T-3's 15" woofers are carrying the heavy load, and they've got displacement to do it rather effortlessly.
I've tried a near similiar, quasi-DBA in my own setup running the EV mains close to full-range (high-passed at 40Hz) and moving the right channel TH sub to the right corner behind me for a total of 3 bass sources, but even after a lengthy fiddling with delay/polarity, gain, and cross-overs I never came to a satisfying result. My efforts weren't exhausutive, but it became apparent that what was missing was a unified wavefront and the positive effect of high-passing the mains somewhat higher. If I were to try out a DBA again it would be maintaining my TH subs where they are currently positioned and low-passed, and then adding two smaller TH subs (10"-loaded version tuned similarly) placed behind me, symmetrically. Right now though I'm looking into a different sub design, but still a high eff. high-order bandpass, loaded with a 21" neodymium pro woofer.