Why in the world would you even consider two 15" sealed subs in a 7500cuft room? That is obviously not going to give you crap as far as real bass with movies and music! If you are dead set on keeping these sealed enclosures then you need to order a MiniDsp to boost the low end and EQ the response before even posting another word about this setup, otherwise you are severely limiting what you could have achieved if you had done this from the start. If it were me, I would ditch these sealed enclosures and go with a pair of ported enclosures tuned to 18hz to 20hz. That would make a world of difference, so much so that I highly doubt you would even consider going with the 12" JL.
This whole trend towards sealed subs that had been going on for the past year seriously baffles me. In my opinion, the only people who should be using sealed subs are those with the LMS-U or UXL-18's, and no one with a large room! Ported has so many benefits!