My understanding is that M80s only dip down to 4 ohms, so they don't fall into the "badly behaving" class of speakers HK mentioned. HKs seem to be pretty good with 4-ohm speakers (specifically M80s) -- I haven't heard any problems.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8