i only use my stereo on sundays for 6 hours. i recently started to put paper towels above the air vents on my pre amp tuner and power amp to keep the dust out. i have a mcintosh mx132 and mc602. is this a bad idea? i know heat is the worst enemy for electronics but i don't play it loud anyway!
I own several of these and they are very effective.
To be used only when your electronics have cooled completely. In other words...when not in use and powered off. As stated above, never a good idea to cover vents during use. With anything...

You can make some covers from silk or find black coloured fibre cloth to cover when not in use.

    Use these for all my equipment when not in use. 
Will last a lifetime.
there are other companies, not sure the ones I have, but they work great!

"...You can make some covers from silk or find black coloured fibre cloth to cover when not in use..."

I use cheap black towels from Walmart. 
Silks in the summer and leave them on or fold them back. The winter is the same way. 50 years next month with Mcintosh. Nothing better.. If I have the cages in place I’ll leave the silks in place in the winter and just flip an end for a little air..

Change the colors for the seasons.. I have silks over 75 years old I’ve had for 45 years.. Woolite and a 5 gallon bucket once a year. Won’t scratch or hold abrasive material, it’s SILK...

Make sure you do not run them with the cover on or you could hurt something in the long run but you can cover them any other time you want to.
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.
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.
Our system is within a few feet of our wood burner. The audio system has cooling (Infinity) fans where needed. But we went through two TV's. Big screens. A Panasonic and a LG. Finally quit using the wood burner for anything other than coziness. No more heating that room with the wood burner. It's nice in these Ohio winters, but electronics and heat don't mix. At work, we had fans in all our electronic cabinets to try and keep the heat to under 100..And this is industrial equipment...    
All electronics fail first due to mechanical fatigue. This can be caused by several things, vibration, hot and cold thermal cycling and intermetallic formation.
I use covers for each component when not in use.  Easily removed for playing time and easily put back into place after playing time (but only do so after the equipment has completely cooled down).  In order to have a nice look of fit & finish, I went to a local yardage shop and picked out a light weight and nice color of soft velvet.  I gave them the specifications for size that I wanted for each individual component (components vary in size).  They cut the pieces and sewed a box stitch around all edges.  These look as nice as the purpose and function they provide.  And low cost, too.  I hope this and other comments above help you in your quest.

Number one most important comment in the above posts, that you absolutely must pay attention to, is NEVER to play your equipment with a cover on it.  Ventilation during playing time is essential to product life, if not survival, including fire hazard, which could mean your own survival.
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)...;)

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.
You should leave amps on all the time. What stresses them is turning on and turning off - the surges in the system which reduces their life (I am talking solid state here - I know nothing about tubes). Many preamps and most phono stages have no on/off switch because of this.

If you have an exceptionally dusty environment, maybe get some type of air purifier, although will create noise in the background, which we all hate.
Well, he only listens about once a week and this is a 600+ watt/channel Mac. It is very well designed and can take the on/off without a problem. My SS amps have been switched on/off for 42 years and I haven't had a problem, knock on wood. His McIntosh kills my amps!
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Even if turning it on and off doesn't break the amp, it will cause premature wear and sound degradation.
i have my mx132 on standby. the red light is on. i have my mc602 on remote with it's red light on. can i cover the vents and heatsink when i don't use my system?
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