Is this too hot for my preamp? What should I do?

Heat kills....if you can get a rack with a shelf for each component it goes a long way toward a long life.   

Staking is very bad  don't do it put them next to each other.

I always think of monoblocks sitting right next to the speakers.  

This brings up a good point. How hot is too hot?

I have my SS Levinson amp and my AR LS-16 pre in a console I made myself.

There is ample room to the sides and rear and good room above each component. The amp has holes under it for drawing cool air. It stays quite cool.

The pre gets hotter and did so even when it was sitting on a top shelf with nothing above it. Everything is fine now but that may change in the summer when room temps are warmer.

So just in case I’m looking at some cooling fans made to go in component racks. They are pretty cheap and they say they are quiet. You can get a temp probe that runs a thermostat..............but you have to know where to set it.

So what temp should be the cutoff? Where should that temp be measured?

There is a difference between an amp warming up electronically and it being heated up such that it is warmed up from external sources.