Maybe I read this wrong but, check the following review out;

http://www.audioholics.com/productreviews/avhardware/Denon-AVR3805_review04.php

Correct me if I didn't get the right info from this review, but if you are not using the 2 rear amps to run the rears and instead use them for half of the L/R fronts, at a minimum would you not reduce the heavy load draw on the amps normally regulating the main L/R speakers from the 4ohm, M80's. This in turn would allow you to provide cooler running on the original L/R amps, sharing the load.

M80 VP150 4-QS8 EP600 Monitor Audio S8 NAD T 773 Anthem MCA3 II Hitachi 57" projection