My setup is a front projector witha riser against the back wall. Currently I have a center speaker under the screen (angeled upwards) and a front right and left at ear height. My right and left surround are ceiling speakers (different make), and I have no rear center. Two questions, how much difference does it make having the rear surround on the ceiling vs walls; and how could I place a rear center when the couch (on a riser) is against the back wall? Should I mount a speaker on the wall above the rear couch? Will it matter that teh front center is on teh ground pointing up, and the rear center will be high pointing down? Should I use a ceiling style speaker and mount it in the wall behind the couch? Thanks.