Mc501 and Powerguard--Have you activated it??

What binding posts are you using? If you have the speakers hooked to an impedance than higher you will overheat and the sentry will shut it down.

Are your speaker cables grounded properly, the only time this happened to me was when I connected a spade upside down and part of the ICONN the spade was screwed into was touching the mounting plate (MIT Cables). Otherwise I don't think I've come close to full power, even at 60+ on my volume.

Same speakers as you mentioned with a Mac 500 tube pre along with 501's. I was at a friend's home during the summer, an all day event and the speakers were at pretty well concert level and no issues what so ever and when I actually touched the top of the 501's they were just warm. I actually owned this specific pair prior and played them very hard at times with Eggleston Andra's and never had any issues you are describing.

Something is up, I would check your connections first assuring that you do have the right tabs hooked-up as per your speaker load, also check to insure everything is secured and not touching anything.

Do you have these into some type of power conditioner, both mine and my friends plugged directly into designated lines direct into the wall.

By the way my friend now has moved-up the line to the Nova's and I was over at his place a week ago having a listen, no issues what so ever and we were cranking the system. They are set-up in a very large open concept and the main floor area of aprox. 3,000 sq. ft. just to give you an idea.