Hi there, this discussion caught my attention in my weekly update. I have dabbled in this space and have a few points to consider.
Its necessary to consider the desired outcome, if it includes accuracy attention can be given to many things, for example minimising 'noise' in digital chain typically improves bass, and will reduce pressure loading in the room. Power conditioning and cables are very important which is expensive when setting up distributed subs. Subs fuses matter too. 1 well implemented sub might be better than 6 done poorly.
Line arrays and distrubted bass drivers are common in large venues and there's plenty of science in that field to consider. They rely on managing/coordinating specific placements, delay, and active crossovers. Complex compared to most home 2 channel systems, and also they're not focussed on a single listening position.
Integration with main speakers is critical. I use active crossover including to roll off mains.
Placement of subs in the room is important, next to the mains might not be ideal, I've found against the wall behind the main speakers better, and a specific distance from the corner.
RELs approach seems adopt the simplest set up I assume to not put people off (from buying more subs).
You can do arrays with any subs. However, it's probably easier to use identical subs than different subs which might have different phase and group delay (I have subs with 4ms delay from input to output, and others 12ms, this makes it awkward to coordinate phase and delay with the 12ms on top of the 4ms - same pair each channel).
The benefit of having a stack of three subs might not be that there are three, it might be the placement (height) of them in relation to ceiling height and pressure zones that matters. I use a rack (from hardware store) to place my subs specific distances from the floor).
I hope this is helpful.
