Hi OmahaBlu,
Very strange. I have lots of questions and want you to try some different things if you have not already.
So is the left M80 "cutting out" randomly and does it come back on randomly or after it "thumps" it stops producing sound for the rest of the movie? Are you hearing any other sounds like "crackling" in the left channel? After the left M80 stops working does adjusting the volume control up or down cause the left channel to start working again? Are you running HDMI or multi-channel analog to your blu-ray player for sound.
What crossover point are you running your M80's, are they set to large? Have you checked each tweeters and woofers if they are working correctly? Check your speaker wire as well for any loose connections at the AVR or speakers and any damage to the wire with left M80.
Good luck.
