Does the Denon have a fan of its own ? If not, one thing that might help is raising the reciever up an inch or so. It would give you less clearance at the top but more clearance at the bottom so I *think* you would get more overall airflow.
The other thing that would help (if you haven't already done it) would be making sure there are no lips to trap a pocket of warm air. The ideal would be if you could remove any lips at the rear but keep a lip at the front so the air flows out in one direction. The big thing you want to do is minimize turbulence so the air flows smoothly... (or, as you say, a hefty fan works well too
).
