How do you figure? Firstly the amp doesn't get hot. Secondly if it did there is more heat transfer with the amp on its side. He keeps it that way because of the internal dimensions of the cabinet.
Amplifiers rely on convection cooling that is why the fins on their heat sinks are vertical not horizontal. Probably not a problem but I wouldn't do it. The amp was designed to be horizontal and draw cool air in through the openings in the bottom cover and expel the heated air through the top.
Andrew Marshal commented on some heat in his review but I won't have the time to find out for myself for a few days.