Having owned the BG-X system, both with 2 and 4 subs, and having owned other 18" and horn-loaded subs (in both the live audio context and in the home theater context), I will say the low frequency response is impressive for 4" drivers. With a system comprised entirely of BG speakers, I have had numerous people (with high quality systems themselves) rave about the quality of the sound. My issue, and it seems like the poster's issue, is the headroom - particularly during explosions, etc., on some of today's LFE intensive, highly dynamic movies. If I had another 2-4 db, I think I would be wholly satisfied with the 4 sub setup in my 17x13x9 room. With 5000 cubic feet, you may need 8 of those things!
While BG states that the woofer surface area of the 4 sub system is the same as two 18"s, the problem - in my opinion - is the lack of excursion in the 4" woofers as compared to larger drivers which can simply move more air. The speakers hit a hard clip, and when they do - the flub sound is very obvious and quite irritating during an intense passage.
I have considered adding a freestanding sub to augment the bass, but I think that may lead to additional issues in integrating the subs. Or I could sell them all and buy 2 very nice in-room subs and find room for them. The other solution is to add another 2 BG-X subs, but I'm not sure that will make enough difference.
As an aside, I get a fairly flat in room response down to about the mid 20 hz range, according to the ARC on my anthem receiver and REW. Not having any null issues at this point. We played with location prior to install.
Just food for thought.