Nice ideas, good stuff.
The Deltalite 12s would be likely be great.
I'd approach this knowing the loudspeaker and room function as a system.
edit; I wanted to come back and preface this by saying ... you may full well already realize the significance of said approach
A loudspeaker system (mains, surrounds and subs), is only going to be a good as it's environment, and the set-up prowess of the operator. The room absolutely dominates the sound from the transition frequency, downward. So, in addition to the speakers/subs, plan for some treatment, and modest investment in measuring gear, a multi-sub processor of some type with manual capability to manipulate in both freq and time domain.
I'd probably go with four individual subs, instead of two-dual opposed, for increased optimization potential. In that line of thinking, I'd come up with some comprimises somewhere so that a significant effort could be put forth into room acoustics. I'd incorporate as much bass trapping as the room can aesthetically handle, and determine locations for early/first reflection attenuation at both the ceiling and sidewalls. Contemplate entirely absorbing the front wall area, at least where the mains are, incorporate the rear cabinet/wall unit into a diffusion/scattering approach.
If you absorb, you absorb it all. Ideally, a minimum of 4", w/a 4" gap,...otherwise you're merely filtering the reflected energy. Thus, you create a lifeless, dull, LF heavy reflection. So you either absorb it all, or none at all. Just don't filter the reflected energy,....very common, and easy to make mistake.
I'd go with rigid insulation if you're using panels of 4" thickness, all up to 8". From that point, I'd likely switch to fluffy insulation (cheaper and outperforms rigid at these thicknesses). Now, by all means if you need the rigid for structural reasons, go with it. Be mindful of the gap, and maximize effectiveness.
It's all about the room, the room's all about the time domain. A well optimized, well damped LF response from ~200hz on down, is wonderfully enjoyable, articulate, tight and tuneful,...just nothing like it. The vast majority of otherwise very competent systems I see around here at AVS, cannot possess the high clarity of a properly damped loudspeaker/room system.
I know the acoustics isn't as fun as picking out the speakers and subs, but it is the limiting factor.
Best of luck
btw; 7 posts in 5 years,...cool. I lurked for a few years before I ever joined up and began contributing.
Flat, Deep, Clean, Linear, and Loud
Active 16.8kw, 7.3 system
(3)Seaton Cat12C up front, (4)QSC K8 sides/rears
(2)Seaton SubM-HP, (4)18" IB