Any harm in using an amp with the face down

Check with the manufacturer or a knowledgeable dealer. It is OK to do what you're doing with some amps, others not.
Many times the ones that it is OK to mount other than "normal" will say so in the manual.

as far as i know even tubes can be placed in arbitrary position. from the way the amps were designed i've seen the elements placed onto the number of different positions with no compromise. on guitar amps many times you can see tubes placed upside-down. on high quality amps very often you can see tubes placed sidewise to save the space.
BUT
there are could be transformers and coupling elements that need to be placed right otherwise they might hum. the toroidal transformer and low-capacitance capacitors basically can be placed into any position while the large coupling capacitors, power and output transformers, coupling transformers need to be placed as mounted by manufacturer.

just to get off topic a bit further - as for tube mounting, it depends. Big high-heat tubes (845 for example) only can be mounted vertically. It spec's this in the tube manuals - the RCA manual is a good resource if there's any question. Other lower power tubes - sure, sometimes they are mounted horizontally/etc. There can be heat issues doing this though - sometimes dissipation is not as efficient. Preamp tubes - mount however, they dont' generate all that much heat. more heat = less life, when it comes to tubes. (within certain limits of course).

YMMV,
-Ed

Depending on amp specific you're dealing with there can be convection cooling interference that might lead to decreased cooling / higher working temperatures. Also consider that resonance / vibration issues are affected by say footers and placing the piece as mentioned might render a different sound presentation. Another $.02

Thanks for all the usefull input. I'm gunna run with it.. Take care. E974