Thanks all for the helpful thoughts. I will try to post some pictures of my system.

Part of my dilemma rests with the PS3 and my DVR that sit on the same rack. The PS3 only runs when I use it and I have it set to turn itself off after a certain amount of inactivity. But the DVR is on 24/7 (of course) and always runs very warm. So what I keep debating in my mind is whether the DVR should be on top so that it can't warm the electronic components above it. Right now, my AVR sits on the bottom and never gets past warm. I must admit, I'm surprised that the 990 runs warmer than the 7500 in Mahansm's system. I would've thought an amp would run warmer than a pre/pro.

I think the original question of this forum relates to electrical interference between components based on Gooomz's question about distance between components, but mine is a question of heat. The question I have in my mind is whether the warmer components should go in the bottom of the rack or the top?