As has been mentioned before, the impedance setting only cuts the power output and as you have discovered, the 663 is more than capable of running the M80s in standard mode. This alone should be evidence enough that the Emotiva has an issue. I would try to get them to send another, if nothing else just to see if it is just that one amp.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70