Mutts, the first point is that you don't have to expect "diminishing fidelity returns". There's very little correlation beween price and sound quality in audio electronics and any of the combos that you list should perform transparently for you without adding any audible sonic character of their own. The small differences in maximum power capacity(2-3dB)aren't likely to be a factor.
My suggestion, somewhat along the line of Mike's, is to first get a powerful receiver such as the Yamaha 2700. Try this first and you might well find that it meets your needs entirely, while saving a significant amount of money for something more important. If you'd find, however, that you'd want something still more for the M80s, you could buy a separate two channel amp or two monoblocks, and use the 2700's excellent processing together with its amps for the center and surrounds. But at this time it shouldn't be assumed that it would be necessary to do that.
