Running them full-range won't hurt anything. All that matters is what pleases your ears.
Lots of people get really hung up on letting the sub do all the heavy lifting. I'm a little less convinced for a number of reasons. First, you've got amp headroom in spades so you don't need to worry about how much power it will take to produce low frequencies on the M80's. Second, there can certainly be room nulls in the 40-80Hz range so having multiple sources for those frequencies (both the sub and the mains) at different locations can be of meaningful help in evening the overall bass response of the system. Third, the M80's have very useful output down to about 40Hz so you're not asking them to do anything they weren't designed for. Finally, my experience in my room is that I prefer to run my M60's with a crossover lower than 80Hz.
Experiment, find what you like, and enjoy yourself.
And - if you're not 100% satisfied - I think you should call Axiom and give them the opportunity to make it right.
