Were all the speakers set to -7? That shouldn't happen. What was the sub set to? If the sub was closer to 0, or positive, try bringing up the volume on the sub slightly.

It won't be reflections that makes the receiver trim a channel, but just the efficiency of the speaker. I don't have M80s, just '22s, so I don't know if the -7 is appropriate, but it does seem like a lot.

Also, what was your final master volume setting that you considered "cranked"? I listen to films at -15 dB all the time.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris