The real enemy is condensation.
Just like a cold glass of brew on a hot day, the cold chassis and electronics in a warm environment will cause moisture in the surrounding air to condense on the cold surfaces.
It should be obvious that moisture and electricity don't mix well.
It's tempting to fire up the amp to speed the warm-up process, but it would not be a smart idea to do so.
Give not into temptation, my son!
Condensation is created when there is a 17 degree celsius diference between the air and the surface tempature of whatever is in question. Being on the road in the winter is the issue, it really doesn't take long for gear in the back of a truck to freeze when driving so loading in and set up typically take up to two hours and anytime after that comes sound check. I guess my biggest concern would be the tubes maybe but I would have to figure a few hours should be ok for the electronics and speakers to get to playing temp... Just wondering if Mesa has any offical advice on this considering they have countless rigs out their right now in the cold.