Since I still didn't get an answer, I will ask this again...
We're talking in theory right now, just so everyone knows. I may not even use IB3's...
But let's say that I do use IB3's... They mount 9.5 inches deep, so I would make the wall 1 foot deep. The wall is over 10 feet long, and 10 feet tall... That means I COULD have like 100 cubic feet of box space. I am pretty sure that would be enough for 2 Fi IB3 18's, correct?
Also, the subs would be behind the viewers. So would it be better to have the subs:
Low, where they would be blocked by the couch?
Medium hight, where they would be aimed at about the back of your head?
High, where they would be almost touching the celing?