Check out the custom fan boxes made by acinfinity; they are incredibly inexpensive ($100 or less) VERY well built and are superb at solving the problem completely.
I have done stacking like that. The amplifier heat will rise and heat the HT Processor. What you want to do is get a piece of 4x4 lumber and cut 4 pieces into something like 1-1/2" or 2" thick slices. Then cut a 1/4 plywood to set on top of that. You can paint/finish/glue these together. It should look like a table top with 4 stubby feet underneath. If you want, you can pad the bottom of the feat to avoid marking the top of the amp. Then put this between the HT Processor and amp. You can use other material for feet, the 4x4 is just an example, but it's important to put a couple of inches of space above the amp for heat ventilation.
This becomes like a heat shield for the HT Processor. I have found it very effective.
Umm... RTFM... Allow at least 6 inches (15.24cm) above the top...
"Allow at least 6 inches (15.24cm) above the top, 2 inches (5.08cm) below the bottom, 3 inches (7.62cm) behind the rear panel and 2 inches (5.08cm) on eachside of the Power Amplifier, so that airflow is notobstructed. Allow 2-1/2 inches (6.35 cm) in front ofthe mounting1 panel for clearance. Be sure to cut out a ventilation hole in the mounting shelf according to the dimensions in the drawing."
Refer to drawing for further details...