How hot is too hot?

Your amp is class A/AB: it can getting warm. A Perreaux  gives ( on the display) 65°C and may not be hotter than 90.  After listening switched it in “off”. The warming up will be 30-45 minutes.

I run Class A.  I wouldn't leave it on when not listening.  The heat will shorten the life of components, particularly capacitors that will dry out.
I wouldn't bother with a thermometer, if it's too hot to hold your finger on it, that's too hot.

Once again. I have the Hint 6. It stays on 24/7. It comes nowhere near to being hot even when I'm driving my Dali Opticon 8s at higher spl. I venture you have something wrong.

Heat is normal 6" clearance is very bad should have at least 1foot.Put it on a stand which will be much better for heat dispersion.It should be shut off when not in use 1/2 hour before listing turn it on.Good luck.

epz, the Hint 6 is an excellent unit. It has a thermal breaker. If it gets too hot it will shut down. If it is not shutting down you are OK.

clearthinker, I think in general you are right. My old Krell KMA 100s were class A and burned out after 20 some odd years. Components are much better now and more heat tolerant. I expect their life span to be longer. 
It seems all the amps I am interested in run hot. In an attempt to improve their life span I use a fan system which you can see on my system page.
My approach to the fan noise was to put the amps in the room under the speakers on a shelf against the ceiling. I did have to put a potentiometer on the fans to slow them down. At full speed I could hear them upstairs.