I have to correct RobR a bit here.
Axiom does not PROMOTE the use of a receiver they have not tested with the M80s and they should continue to do so. Obviously they cannot test every receiver out there with their speakers, they can only recommend the receivers they know for sure will drive the M80s, but that doesn't mean other receivers are incapable of doing so.
I own an Onkyo DS797 and it has driven 4 ohm speakers no problem.
Do not rule out receivers based solely on the suggestion of knowns from a single person. If you pose the question in forums and hear from ppl who own a combination of receiver and M80s you are interested in, then you should have your answer as to whether it will work or not.
More often you will see that the receiver companies will not quote their equipment as being able to run a heavier load so they can protect themselves from having alot of returned merchandise from over zealous customers pushing things too hard. Even some of the receivers Alan has mentioned have a listed 6 ohm max limit in their manuals, yet they still work with the 4 ohm speaker.
As long as you buy a quality unit and don't skimp on the $$ too much, you are
more likely to have a unit that is capable of running the M80s.
As for the M80 impedance itself, see
this graph for an idea of how much the speaker averages at the 4 ohm level.
