I'm now stacking my Denon 5900 disc player on top of Parasound HCA-1000A amp. I pulled out 4 of my custom made (I made them years ago) wood blocks to provide spacing. The internal heatsinks of the Parasound are still causing temps to rise inside to 110 degrees F or greater. Not good. That is with about 1.5 inches of added spacing from my wood blocks, or about 1.75 inch between the top of the Parasound and the bottom of the Denon thanks to the Denon feet. Check the parasound manual, and it says that 6 inches are needed inside a rack with the front open. My stack is freestanding so it doesn't need 6 inches, but probably at least 3.
Most of the solutions described here provide about one inch. I suppose I could stack 3 hockey pucks (3 looks doable, 4 looks dangerous). I went to a Michaels craft store and they had 3.5 inch blocks (but sadly, the store I went to only had 3 in stock, and the Denon needs one on each corner I think). Now I see online stores with 3, 4, and even 6 inch cube blocks. I'm going to try the blocks I have (and get another one from another Michael's tomorrow for that purpose) but if it doesn't work well I'd step up to 5 inches, which is about as big as I'd want to get because the cube also directly blocks airflow.
http://www.craftparts.com/blocks-squares-wooden-blocks-cubes-wooden-c-209_212.html