"To thine own self be true."
You've got to ask yourself, "Even if these M60's are fantastic, am I going to play the 'what-if' game"?
For me, the answer was yes. I knew that I just had to have the M80's because otherwise I'd always wonder what they were like. I knew that if I bought M60's, that in 6 months or a year I'd be right back on Axiom's site trying to convince myself to order M80's, and then stuck wondering what to do with a pair of M60's. A year later, when the $300 is irrelevant, I'm still absolutely thrilled with my M80's. The M60's are fantastic speakers too, but would I be as happy? Knowning myself, I would not.
That said, are you 100% sure that the Onkyo won't work with 4-ohm speakers? Onkyos usually have very robust amplification sections, and I thought that Onkyo (and Denon) were two of the most recommended brands for driving M80's. I could be wrong. Keep in mind that the M80's may set of an inflamation of upgradeitis. It did for me.
But as the other's have said, since your M60's have already shipped, you might as well set them up and listen to them. You've got 30 days to make up your mind. You may decide that they're all you'll ever want.
In my case, I gambled that my c2002 Pioneer Elite AVR would be able to drive them. It could not at 'loud' volumes. Clipping and distortion. Another $500 to Emotiva solved that problem.
