I have front and back IB 'ceiling' manifold install's. The front IB holds 4 x 15" drivers, and the rear manifold 2 x 18".
Using REW, I found reversing the front manifold drivers phase, and keeping the rear manifold drivers in phase with my main speakers gave best results. I then ran Audyssey (1 in 2 out) to combine the two manifolds together.
You certainly will get a lot more bang for your buck going DIY..