You're forgetting one important factor, there's air inside of the amp.
When mounted upside down, the heat can't efficiently escape the circuitry into the heatsink/cooling fins.
But... If you don't believe me, feel free to ask the couple of people who have fried an amp mounting it in this manner. :p
I'm not saying that it can't be done. Some amps have sufficient cooling capabilities or never even generate enough heat to become a factor. Or, they might have great thermal protection circuitry which will shut down the amp when it gets too hot. However, as you start decreasing the impedence load and/or driving the amp closer to its limits, it will generate more heat, which is the ENEMY!!!
"As they say hot air rises and heat sinks"
How is this possible? Could you explain please? Sorry, never got into thermal dynamics with my accounting degree.
