I do keep my garage gear out there all winter, detached so it gets down to -10° sometimes and can sit for long periods under 10°. I am a bit worried about cracking a solder joint. Not the I turn it on out there below maybe 50°, but I still worry about the old solder joints expanding\contracting, maybe at different rates than surrounding board\traces?
LOL< now that I've said that, it seems like it shouldn't be a worry. Not worried about speaker surrounds, although it is crazy dry during winter. You guys said moisture? From where? You'd have to have a surface colder than surrounding air, to drop below dewpoint to generate condensation - and that ain't happening unless your introducing moisture (oh, attached garage, do you get moisture from the house?).
I don't think I'll worry about it.