What are the signs of tube preamp overheating?

I have a tube preamp I didn't build. The cover it came with did not have any vents and I was concerned about overheating so I got another cover that has slots on the side for venting. After a while the cover really was quite warm...not too warm to touch but warmer that I thought it should be.

I could have sworn I heard the music I was listening too becoming distorted. Is this a characteristic of an overheated tube preamp or amp?

I took the cover off, let it cool down and everything sounds fine...even after listening for 4 hours.

I'm thinking I should probably add more slots or holes to the cover on the top.

It's in the nature of tube gear to get hot. Tube amps in particular can get very hot. Tube preamps too will get warm under normal conditions. So long as you're not keeping them inside an enclosed cabinet they should be fine.

My PAS2 and Counterpoint SA-1000 preamps run cool. It would be hard to tell if they are on or off purely from the cover temperature. What tube types are in this hot preamp?