The concept of vertical mounting of a passively cooled device is that as air is heated it will move upwardly along (or across) the cooling fins, even in the absence of external air flow such as an open window, A/C vent, or fan, i.e. convection.
Ideally, the amp should be mounted so that the fins extend vertically. One of my amps was too big, so I mounted it on the back wall with the fins extending horizontally, with no apparent ill effects.
http://www.electronics-cooling.com/h...alccorner.html
http://www.frigprim.com/articels2/parallel_pl.html