Mounting your amplifier is entirely dependent on the heat-sink design. If one side of the amplifier IS the actual heat sink then THAT side must be up at all times. If the sides of the amplifier are the actual heat sink then you can invert it all day long and the heat exchange will be exactly the same.

Your biggest concern is whether or not enough fresh air is running across the amplifier heat sink and making sure that the heat travels AWAY(up) from the circuitry.