BBC, the first point is that there're only seven amp channels available. Next, both the A and B surround outputs are for side(not back for 7.1)surrounds. As Randy discussed, some prefer direct radiating side surround speakers and others prefer wider dispersion surrounds(e.g. QSs). The A and B outputs allow the use of both types, either individually or together in parallel(still using just the two 4th and 5th amp channels). The 6th and 7th amp channels can be used either for back(not side)surrounds or they can be assigned to instead drive two main speakers in another location.
Using one of the B side surround outputs to drive a back center speaker would only send left or right side surround material to that speaker and wouldn't likely be a satisfactory effect. Use only the second zone connection or the back center, but not both.
