One last point -- seems like pretty much every receiver and amp can drive 6 ohms without any problem. You will get a bit more power than the factory rating; potentially you could get up to 30% more power but I would be thinking about 10% or so more at the most.

I believe M80s are 4 ohm so that they can make the most of big honkin' power amps with heat sinks the size of a small dog. For a normal receiver I wouldn't plan to take any more than the rated power into 8 ohms, and even then the receiver will be running a bit hotter than normal.

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