Mc501 and Powerguard--Have you activated it??

Today I was litening to a live Grateful Dead show from the early '90's--"Without a Net". I am using a pair of Mc501's with a c220 pre and a pair of JM Lab Alto Utopias. After 25 minutes of driving pretty hard and really loud(volume level 67 on pre), the left amp's Powerguard was activated.
When I got up and felt the amps--WOW!! They were HOT,HOT,Smokin' HOT! To the point that I could not touch them for more than about 2 seconds.
The good thing is that they were doing their job by shutting down, however I NEVER heard the JM Utopias sound as though the amp was clipping. They never sounded strained.
I was wondering if anyone else with Mc501 amps had similar Powerguard experiences. How hard do you drive till they shut down?
BTW: the amp "came back" after about 2 minutes.