Actually Cajunpepe, what you triggered was the conventional thermal cutout circuit, not the Power Guard per se. In addition to the actual Power Guard circuit, McIntosh frequently ties both this thermal cutout circuit and a DC-offset protection relay (on direct-coupled amps) to the "Power Guard" indicator light. The operation of the actual Power Guard circuit is noticed when the indicator lamp flashes with the musical peaks, when the power meters are around a full-scale reading, not when the amp mutes itself for an extended period. In any of these conditions, damage to the amplifier (should be) very unlikely, and you won't notice any audible clipping. But when the amp is driven very hard into the actual Power Guard operation, you will hear some marked dynamic compression.
Mac solid-state amps, like any conventional Class B circuit, will generate the most heat when the average power output is about 1/3 of maximum. When the designers decice exactly how much heat-sink area to allow for cooling the output stage, there's some educated guesswork based on the peak/average power ratio of musical signals, and the amplifier's anticipated operating conditions, in terms of power output and ambient temperature.
Sounds like in the case of the MC501, McIntosh cut it a bit close for party use . . .