I'd skip the thermo and just invest in a better fan that runs off your AVR. The bigger air flow, and lower noise you can get, the better. I have several external mount fans and quiet is hard to find, but they cool really well.

Panny 3000 PJ, 118" Carada, Denon 3300, PS3, Axiom QS8, PSB 5T, B&W sub, levitating speaker wire