heat

even if you play your stereo at low listening levels is it still bad to have covers on your units??

"Do you find your nude audio equipment...vaguely 'suggestive'?

PEBTTAC....Problem Exists Between TurnTable And Chair.

Just kidding with a recycled 'puter pun.....sub TT with K(eyboard)...;)

YES it is bad, ABSOLUTELY, NEVER DO IT!!!

Am I making myself clear?

Better yet, contact the manufacturer of your components and ask them.  There is a reason they built slots for ventilation in the component(s).  Covering them up, whether that be on the top, side, or bottom, is going against the grain of the engineering built into each of those units.  I promise you, that is what you will hear from them.  

Time for a Darwin Award- surely paper towel/ filter will only not do damage if there's forced ventilation ie fan. Just put a dust cover over when you're not using stuff and take it off when you are. Mind you I suppose covers can cause scratching on the outside- surely a case of common sense.....

Even if you don't listen loud, the pre-driver and driver transistors are biased high enough to produce significant heat. This heat will continue to build over time during your listening period without sufficient ventilation. The heat sink design for these transistors, most likely NOT mounted on the output transistor heat sinks, will cause ALL components to get quite hot thus lowering their life expectancy.