In my very first electrical engineering course in college, on the very first page of the textbook it read "Electrical engineers have it all over mechanical engineers because they can turn their products upside down without the oil spilling out"!!! From an electrical standpoint; amplifiers can be mounted horizontally, vertically, upside down, etc. From a thermal standpoint, it would be best to mount the amp to a conductive surface, or if not available, so that the fins are oriented for best convection (heat always travels up).
