dmuir
I suggest you read the last paragraph in
this post.
SPL of 100dB (depending on distance) is really quite high. I'm not surprised that your Denon is shutting down considering it is driving 5 speakers to that volume level.
If you prefer to get the sound even louder, you may have to resort to finding a larger speaker since adding 'power' will not add a significant gain in the dB department.
To relieve the receiver of some burden, you could purchase a separate stereo amplifier to run the M60s. However, every speaker still has a maximum limit of SPL that it can attain before the driver hits its excursion point (limitation of the speaker) or begins to distort (limitation of the power source/amp). In your case, the receiver is shutting down to protect the amp in the receiver from overheating but this also helps to protect your speakers from distortion.
Be careful that your receiver is not already creating distortion at SPL under 100dB before it shuts down.