heat

Second the air purifier. 

Don't cover when in use. +3 for air purifier, I don't run it while listening. I just installed one for the entire house.

I run ’puter fans for heat removal, and anything vented has a shelf above it. Prefer practical over pretty.....although can be made so if desired.

I have to be next to them to hear them...

Agree with @oldhvymec's recommendation of silk. I keep my tube integrated covered with a vintage scarf I found among the things in my father's house. It takes seconds to put on or take off, can't create any wear on the amp finish and looks far better than any equipment cover I've seen. There are more than 26,000 vintage silk scarfs listed on ebay, most for less than $20.

Heat can be an issue with electronics for manufacturers that cut corners on the quality of their components. The bigger issue is the heating cooling cycle. This will cause expansion and contraction that will damage components. I leave my gear on 24 / 7. Example my old sansui 7070 running in my wife's business has not been turned off for 24 years except for a couple power outages.