I have Rotel RB-1080 with my M60s. I switched from a Rotel 2 channel receiver (RX-1052) to Rotel RB-1070 and then to RB-1080. While I noticed a great improvement in the first upgrade, not as huge in the second one. I guess it would be more benefitial for you to get RB-1080 since you have M80s.

Axiom M60s, QS4s, VP100 Onkyo TX-SR804 Oppo 970HD Rotel RB-1080/RCD-1072 REL Q150E sub, PS 3