Nick is the resident expert at acoustically-transparent stuff. Check out his 4.0 build thread.

My opinion is that a few inches of space behind the speaker is plenty. And I don't think that having sound-absorbing material on the wall behind them could possibly hurt, and it might help.

Have you considered a vertical M22 as a center? I think that's what I might do if I'm ever lucky enough to be in your position.

Congrats on the projector!

bibere usque ad hilaritatem