Great video!!! Thank you very much! I'd seen the earlier video Jon mentions, but this one clears up a lot of things he spoke about in the previous one.
I come away with my original supposition with a modification in term, that single plane subs are preferred. They also should be of the same type/size, otherwise complications arise.
Unlike DBA, which in its strictest sense is very inflexible in setup, the Trinnov system "will work with what you have", and is more flexible in exact placement - and - improves upon DBA by way of the digital processing which takes into account more of the reflections involved, plus, ALL of the subs are used for producing and absorbing. In a traditional DBA, the front subs produce the sound, the rear subs absorb the sound.
I've been experimenting with various subwoofer layouts for the the last month or two. I haven't had the time to invest in all that I want to try, but the quick point to make that showed up immediately is that, using six subs lined up between the front L&R speakers with a gap in the middle for a couple stacked amps, using the four equally spaced subs created a response that had very little seat to seat variation, but when I employed all six with that gap in the middle the SPL at the MLP became LESS by a dB instead of more. This is due to a lobing effect because each group of 3 subs acted like two subs and some cancellation in the middle at the MLP was created. An effect of "steering".
This is a representation of the layout I've been working with.
I'm not trying to create a DBA, just playing with line source type setups. I tried vertical last year and it doesn't work well in my room. This, being horizontal, and using the floor as much as possible, seems initially to work well at the seating area. These images are a bit of an oversimplification because my speakers are toed in quite extremely and the subs at each end are actually spaced outward from how they're shown, but I made these graphics a couple weeks ago for a friend to explain what I was testing.
Trinnov remains my number 1 choice for processing! It's not yet in reach for me though. Of course, I could've bought one this month with what I had to layout for a new furnace and AC system, so, it's really just a matter of life choices.