As long as the amp is not too big and isn't pushing a lot of power it will cool well enough. Just don't mount it upside down. Most amps have an overall height of about two inches plus an inch or two of cooling space you'll need 3-4 inches of total space to mount the amp. The main problem you're going to run into with mounting anything under the seat are the cross bars that release the latch mechanisms. They are two thin bars(2-3mm) that run from one side to the other. Actually, now that I think about it there might just be one bar. Can't remember for sure. Right now I'm not aware of anyone on the boards who mounted an amp there...very few have used it for a changer mounting location. Look in my photopoint album(link in sig)for a pic of my processor brain I mounted underneath there. HTH.
