Hi SatKartr,
I just had a conversation with Tom Cumberland, our chief R&D engineer who designs all of our electronics, including the A1400.
First, he asked me if you are running a center channel and it seems you are. Do you have it set to "Small"? It should be, because if you have it set to Large, the center can't reproduce low bass and any deep bass routed to the center might cause the woofers in your center to hit the magnet structure, producing the popping noise.
Next, Tom told me that there are several points within the Yamaha where distortion may occur. If an especially "hot" (loud) digital signal hits the Yamaha's DSP processor and if it's so-so, it may not handle the hot signal, causing distortion, which would be fed through to the A1400 and reproduced by the M80s. Another possibility is that even if the Yamaha's processor handles the hot digital signal, the peak in the gain might cause the Yamaha's preamp section to clip, which would also feed through to the A1400/M80s.
Finally, he pointed out that if you don't have a subwoofer and you are listening at levels you quoted of 107 dB SPL, it's possible that the LFE deep bass, which should go to a subwoofer, is going to the M80s and overdriving the M80's woofers so they hit the magnet structure, causing the pop. By the way, that won't damage the M80 drivers--it just makes an unpleasant sound.
As to the Yamaha shutting down, it may well be a product of the DSP board clipping or the preamp section clipping as described above. As JohnK pointed out, the impedance setting should have no effect whatsoever because none of the Yamaha's internal power amplifiers are being used.
Regards,
Alan
