Planning to upgrade from 5.1 to 7.1. I currently have my left and right surrounds (MA Bronze BFX) in bi-pole mode level with my seating position. My seating position is approx. 4 feet in front of the back wall and I want to mount my centre rears on the wall. I've heard that for centre rear channels you are better off going for mono-pole speakers. Is this correct? If so what could I buy to match my Bronze B4s that are wall mountable? If this isn't correct I will probably buy another pair of BFXs. If I do how should the 4 surrounds then be arranged, bi-pole, di-pole, or a mix of both?