I did much the same (Radio Shack cooling fan with Radio Shack DC power supply, using receiver plug). In my case, the receiver is inside an armoire wih 1" of clearance above it. With the door closed, the inside gets really so hot that I worry about damage to the CDs stored inside, plus the possibility of damage to the furniture itself. (The armoire is Ethan Allen and costs more than my sound system combined!) So I put a fan sucking air out of the armoire itself. But the air circulation is pretty lousy, and eventually I gave up and now just leave the door open and the fan off (the fan isn't absolutely quiet and I'm really picky about noise). The way I figure it, the receiver has lasted 3 years already without failing, and if it dies from overheating tomorrow I'll just go buy a better one. Electronics will die sooner when they run hot (many degradation mechanisms in ICs are accelerated rapidly by temperature, often something like a factor of 3X per 25deg C). But I'd rather enjoy the sound on a short-lived receiver than have noise for a very long time.
