I'm not sure what the problem is with this? i'm willing to spend an extra $400 to test out the speakers, rather then damage a vintage amp which i know works.

The QSC will handle the shut downs....IF that happens. The old Onkyo...I'm not sure about. Plus i'm not willing to risk a vintage amp that at the time cost as much as the M80's. So what seems to be the problem?

It's pretty basic...if the M80's shut down the QSC.....there are broke. Period. The amp was built to run 4 ohm speakers.
If they work...great...problem solved.