Hey Jab,
I was one of the guys to answer your previous post. Cold by it self is no problem, but another poster (stokes?) said that bringing a cold amp into a warm room and firing it right up could cause condensation issues with chassis components. That makes sense. If there is a lot of humidity in the air it might be a real good idea to let the amp warm gradually. That is another reason to have an amp covered. In a pinch one could turn on the tube heaters and leave the cover on to hasten warming the chassis up. I would not put the standby switch on though, just use the heaters. I have several amps, and they are stored in an unheated storage unit that sees down to 10-20 below in the winters, no problems.