As long as you don't power up the cabinet under, say about 10 degrees c, you should be fine.
It's the rapid changes in temperature that do damage due to condensation, especially to the tube.
If you really want to be safe, stick a humidity absorber inside the cab and put a cover on it but I've had cabs survive several winters in a garage without any of those.
If you live in Alaska, then the above doesn't apply. Bring them in
