If you are going to use minimal equipment for the center and rear channels, don't bother. Much of the adverse comment in audiophile circles about multichannel sound results from this practice. The notion that the rear channels are ambience only derives from early matrix multichannel experiments, where it was discovered that ambience in stereo recordings was often out-of-phase, and would isolate nicely to rear speakers.
Furthermore, long before there was discrete multichannel, I came to realize that the Center front channel, which in a stereo recording is signal that is in phase for the two channels, constitutes most of the signal. In particular, soloists are almost always recorded equally in the two stereo channels, so that in a stereo system they appear as a phantom source in between the two speakers. In a system that uses a center channel speaker, the soloist will be reproduced by that center channel speaker. The soloist is certainly important, and center speaker quality is crucial.
Furthermore, long before there was discrete multichannel, I came to realize that the Center front channel, which in a stereo recording is signal that is in phase for the two channels, constitutes most of the signal. In particular, soloists are almost always recorded equally in the two stereo channels, so that in a stereo system they appear as a phantom source in between the two speakers. In a system that uses a center channel speaker, the soloist will be reproduced by that center channel speaker. The soloist is certainly important, and center speaker quality is crucial.