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.
I currently have no sub. The 3 fronts are set to large and QS8's to small. Yes, all speakers were set from -6.5 to - 7.5 db under MCACC. With my old system -30 to -25 was very good, -20 if it was a quite movie - this would be very loud for most movies though! When I used the MCACC setting I had it closer to -10 before the sound was comparable to my old "loud" setting.
