Kay,
The Ohms rating on a speaker effects how diffucult it is for an amplifier to push current through it, or how "resistive" of a load the speaker is. A 4 Ohm rating has the potential to be a problem for lower end amplifiers and could cause them to shut down. The M80s are a fairly efficient load and are said to be an easy 4 Ohm speaker to drive.
That being said, it seems that the Denon, H/K, and Yamaha receivers can be used with the M80s without any issues. I have a NAD receiver that drives a 4 Ohm load with no issues, and would also add Arcam and Rotel to your audition list if you have the budget.
I can't help on the M80 versus M60 issue, but if you plan on the M80s, stick with a good name receiver and you should have no problems. Regards,
