heat

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.

Dating myself here. I bought my JBL Dorian S12 new at Allied Electronics on south Western Avenue in Chicago.

Our cat liked the grills. Remember grandma and her furniture covers? I found a company that made that stuff and had them make 2 clear vinyl flat bottom lunch bag style covers. Slid them down over the speakers. I was happy because I could see my speakers and the cat was happy because he could ‘scratch’ all he wanted. 
Off course I took the covers off when playing music.

Never cover vents on electronic equipment when that equipment is on. Heat is the enemy of electronics (even tubes). Covering vents on operational equipment is asking for failure!

If you want to keep dust off of and out of your audio equipment, buy MERV 11 or above rated furnace filters and change them every 30 days.  Don’t believe the claims on the filter that they last 90 days or more.

They are mainly clogged after 30 days and the dust just blows around the edges of them.