Fans for Home theatre...

[MouseRider]

If the amp is getting irregularly hot, check that your speakers are correctly matched to your amp and that they are not of a lower impedance than specified by your amp.

If you run your amp with speakers that are stated by the specs, it doesn't matter if you push it hard, it shouldn't get unnecessarily hot unless your environment cannot displace the heat that it is generating.

You already have quite an open rack, what is your room's ambient temperature and what's the humidity?

If your environment cannot displace the heat, fans won't help since you'll just be pushing hot air around rather and pushing cool air to replace the hot air.

The other thing is it might not be a good idea to put it right on the top, remember, heat rises, so if it's on the top, all the heat from the other devices underneath it (i.e. 360, PS3) will go up and add to the heat that your receiver is already generating.