The problem with Yamaha is no one is sure that the same P/S and amp that were in the 661 are now in the 663, which is why Axiom has them on the not so good list.

Before panicking I would try it out and let us know how it works so we can put an end to the debate as a few others have looked at the 663 with M80s. If it turns out to not work the NAD will do fine or as you have suggested return the 663 for the 3808 which we know works great.

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