bridgman,
One should also add the idea about hard to drive speakers (not the M80s necessarily but speakers like electrostats). Many receivers do not support 4ohm loads, and even if they do it will tax a receiver more than a standalone amp (shutdown protection functions and such are still SPL dependant).
Not all amps can drive a 4ohm load either, but i believe there is alot more selection. For 8ohm speakers the selection for possible receivers increases dramatically.

"Those who preach the myths of audio are ignorant of truth."