I have two garages, one attached. On the attached garage, tho it is not heated I still kept a stereo out there. Now it does not get nearly as cold here in Tennessee as it does up there, maybe we will get a dip into the single digits overnight, and that is usually for a week or two at most. I would guess our "cold" days average between 20-25 - dipping into teens at night.
Now I do not have my "best" equipment out there. Currently it is a Onkyo A-5 , a kenwood Cassette deck, cd player and a pair of HPM-100 speakers.
On really cold days, I am not out there long enough to use a stereo. On the days that I do use it and it is 'cold" out there, I turn the amp on 1st and let it warm up for 5 minutes or so before i use it (I really do not care about cassette deck and cd player out there)
My biggest concern has actually been when powering down. Always wondered if I should be worried about moisture as it cools back down. Hoping as the humidity is so low it should not be a problem.
I may actually be more concerned about it in the summer as the humidity around here is always so high. I am thinking it may be much harder on the amp when it is 98 degrees with 90% humidity. For now all I can think of that is economically feasible is to insure a fan stays on it