Great advice offered.
In my opinion, the essence of what's been stated is if you're after what an IB approach has to offer, ie., highly efficient bottom octave performance*
, then it would serve you well to implement this into a front of room location. Whether that's in the front wall, front side walls, ceiling, or even the front floor, what's important is the acoustic output must be psycho-acoustically tied appropriately to the LCR mains. Response filling/balancing individual small sealed subs could be employed elsewhere to mitigate the commonplace SBIR issues associated with a sub system.
What's been suggested is even if you don't hit your VAS goals, the hit in bottom octave efficiency can be accommodated by frequency EQ'ing of the proper driver.
Consistent and smooth response over as wide an area as possible is a very desirable trait. PNW's method of addressing height and width modes is very effective by spreading the driver across th e front end of the space. Here
is a decent tutorial you may have already read, however Toole describes width and height spacing of subwoofer drivers to mitigate modal excitation.
a true IB alignment in a typical ~3k cube space, config'd of four co-located FiIB3-18's, modestly powered via 200w/each, can achieve ~125dB@1m@20hz.