Get yourself a fan to help with the exhaust.I initially forgot that I had the carpenter cut some vents into my cabinet to help the AVR, so I don't want to cover those up. I've got about 11.5" from the first vent to the forward face of the countertop, plus I'm ok with an overhang of an extra 1" or so.
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Amazon.com: AC Infinity AIRCOM S7, Quiet Cooling Fan System 12" Top-Exhaust for Receivers, Amps, DVR, AV Cabinet Components: Electronics
In addition, the center channel should sit on small rubber feet, to isolate it from the cabinet (don't want any nasty resonances muddying the sound). The feet will also allow you to "cover" the holes in the cabinet and still allow airflow.
Now you can go with a deeper speaker without affecting the receiver's venting.