Originally Posted by Whiston

You speak the truth.
The purpose of an amplifier heatsink is not to keep the amp cool, it is to distribute the heat away from the devices attached to it, such as diodes and transistors. The heatsink dissipates this heat into the ambient air.

The ambient air is the key word here, the air currently surrounding the amplifier. I will submit that there is an extremely marginal difference in heatsink efficiency, regardless of how you mount your amplifier (within reason), and any thermal shutdown is typically not install related, and more the fact that you're using the equipment well outside the limits of which it was designed.

If that amplifier in that cubby failed exclusively because of it's mounting location, I would send you one of my amps as replacement.

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Furthermore, I'd like to add that feeling an amplifiers for heat is a very poor indicator of how well it is running. As was explained above, given two identical topologies, I would sooner take the amp that ran hotter because it would be the one, theoretically, removing the heat from the transistors better.

(That isn't an absolute statement, take it with a grain of salt).