I looked up a few threads of people using relays, usb, and even pre made kits to cool down a receiver or entertainment center. I was looking for something different and came across thermostat discs like this one.
The positive (or negative?) side of a 12v wall wart will hook up to the thermostat and the other end will connect to a pc fan. When the receiver gets hot enough the switch will close completing the circuit which will turn the fan on. When the receiver is off and cools down the switch will open turning off the fan. Is this correct or am I totally wrong on how these things work?
Only thing I would caution with that non-adjustable switch set at 100 F would be to check the hysteresis of the switch. If the temp it needs to get down to to shut back off is too low, your fans may not shut off during the warmer summer months.