Van your Onkyo, can in fact, run the M80's, it is just a matter of how loud you want them to go. The 6ohm rating is quite standard among avr's. I ran my M80s with my 15 yr. old Sony with no problems. The worst that would happen is the Onkyo overheats and shuts down for awhile. I am not familar with the 607, does it have preouts? If so you could always add a seperate amp to drive the M80s in the event the 607 shuts down more often than not.
Try the M60s and let us know, if you are not happy with them ship them back and order up the M80s, no shipping costs as long as this is done within the 30 day trial period, you only pay the difference between the M60/M80.
