Rockler sells quite a few different vent covers and a powered fan like Ajax mentions. If you determine that heat really is your problem and you don't want to ditch the cabinet, this might do the trick...

http://www.rockler.com/ecom7/product_details.cfm?&sku=6572&cs=1


M60's
VP150
QS8's
HSU STF-2
Onkyo TX-SR805
Audiobytes for the PC