My guess, in a 7500^3 room, in my opinion, you're going find the 10Hz and 12.5Hz is going disappear. I find that's where the rumble and roll is; the tactile sensation.10hz- 104.2db
Of course these are 2m ground plane measurements outdoors. Just imagine what it'll be in room! These numbers also follow closely to what I modeled in WinISD. Pretty close to a Gjallarhorn's output in the 25hz and below area and putting out more in the 30hz and above.
Am I off base with that assessment? My experience is limited to what I've read and learned along the way with REW, three Klipsch subs and a pair of FV15HPs. I don't do DIY. Those FTW 21s will move a lot of air, but will that be enough for 7500^3 in the lower octave?
In room, what are your expectations at 10Hz and 12.5Hz?