I just look at MM9000 specs and I saw it's rated for 8Ohm speaker. The M80 you're using are 4 Ohms and the VP 150 is 6 Ohm. Possible that during the loud, bass heavy scenes the amp went into the overload protection. When you lowered the volume or redirected a part of the power to the sub by setting the speakers to "Small" it went back to normal.