It would indeed be "interesting" if even the 4311 couldn't drive a 4ohm rated speaker, but that isn't the case. Less capable Denon models and similar models of other makes can and do drive the M80s without difficulty.
There doesn't appear to be an online 4311 lab report available, but the similar 4310 tested at over 200 watts into 4 ohms
here .
It should be noted that any LFE in the movie wasn't reproduced by the M80s or powered by the 4311, but by the EP500 which handles the LFE channel, so any shutdown was due to other reasons. The maximum peak in other movie channels is 105dB, which would require about 100 watts, easily handled by the M80s and 4311.
Also, to clarify a point regarding the default higher impedance setting, the higher voltage output allows a higher power output(but still well within the speaker's power handling capacity)before possibly damaging clipping occurs. The default setting lessens the chance of the speaker being damaged.
