I own and rent cottages in one of the most inhospitable temperature-wise places in the US. Temps regularly hit -20F in the winter.
I also own a house (that I occupy) on the property.
I have left lots of equipment out in the barn over the winter. Obviously not my high-end stuff, but lots of CD players, extra preamps, as well as TVs, spare electronics for both the main house, as well as the cottages.
I've also owned boats which had lots of expensive electronic equipment in them (radar, GPS, radios, depthfinders, etc...).
All have been fine over the winters, just let them acclimate before turning them on.
Do NOT put your stuff in a closed closet with a space-heater and leave it unattended. The light-bulb thing (incandescent) is fine (I used to do it in the bilge of my boats, mostly to keep the bilge pumps from feezing up), but that's only going to do so much if the temps REALLY drop.
You state you've been leaving your speakers over-winter there, so no need to worry about the finish on them (cracking veneer, or what-have-you).
I have lots of direct experience with this very subject. Take my word for it, you'll be fine. Manufacturers ship their equipment sealed in plastic bags with silica gel packets as a desiccant. That equipment usually crosses the ocean on a freighter or in the unheated cargo hold of an airplane.
You'll be fine.
