If you mount the amp vertically everyone knows all the bass runs out the faceplate with nothing to keep it inside.
I might have mentioned this before: in my audio cowboy days I happened to be at my dealer when some new gear arrived. We were checking out the new Counterpoint tube preamp, from California. Counterpoint was a visual knockoff of the then famous Mark Levinson preamp--1 U rackmount form factor, which required tube sockets to be mounted horizontally, sideways as it were.
Anyhow, one of the cognoscenti thought he heard a little 'smearing' of the sonic image, when compared with a Conrad Johnson tube preamp (which had it's tubes mounted more traditionally, in an upright manner). His explanation was that the filaments in the tubes were not meant to be heated in a horizontal manner, and that this adverse mounting caused the electrons to 'bend or flow downward', and thus could have been responsible for the slight 'veiling' he (and then we all) subsequently heard.
Of course back then we were all audiofools. So I guess it made sense in a nonsensical way.