I have an Onkyo DS797 receiver. Although the manual has power ratings listed for 4ohm loads, the setup options within the receiver DO NOT.
Odd thing eh?
They have options for setting the ohms to 8 or 6, but not 4.
However, i have not used the M80s on my system but i have driven 4ohm Kefs on it, not that long ago. It ran them perfectly fine at either the 8ohm or 6ohm setting, although putting the setting to 6ohms did start to create some distortion at the higher end of volumes. When running the Kefs with the 8ohm mode, the receiver still ran at its usual temperature without the distortion.
I have heard about some issues with the 787 model, one specifically that is has a fan to help cool the amp onboard. The 797 model does not have a fan and hence, i'm led to believe that Onkyo changed the heatsink or amp with the newer 797 model from what the 787 had. A fan is often used to assist a heatsink that needs the help. With a large enough, appropriate composite heat sink, you should not need a fan (at least not for this price level of receiver).
All that being said, i cannot say to you for sure that the 787 is a good enough model to power the M80s. I have my doubts based on my previous statements and Murphyman's original accord. Sometimes the specs may lead you to believe it will be fine, and sometimes it just isn't.
But when the M80s do come in, i suggest leaving the receiver set at 8ohms and see if the auto protect circuitry shuts it off at higher volumes or extended play time. I would also suggest checking the heat venting from the receiver during this time so you can get a gauge of how well/poor the receiver is performing. The less hot it is, the better chance your protection circuitry won't become a problem.
Make sure you use a good gauge wire as well (16 or less).
"Those who preach the myths of audio are ignorant of truth."