What is the highest temperature I can store an amplifier in without causing it damage. The room is dry but can reach temperatures of 40°C +.
Amplifiers contain capacitors, which have dielectric materials that dry out and break down over time, and high heat accelerates that process. Transformers have lacquered wires that experience the same kind of failures. Polyethylene and other plastic insulation on wires will become brittle.
Storing your amplifier in high heat will not damage it, per se, but it will contribute to shortening the life of the aforementioned components, leading to an earlier maintenance/refurb schedule.
Are these combo units, just the cabinet or just the head? The head, in my opinion, would not be a problem being left in a hot environment. Do you worry about your TV or home stereo being in it? Your car stereo is left in a far hotter state if you live anywhere south of Alaska in the Summer. I would worry about it more in a situation where, if you moved it, condensation would result as it would not be able to acclimatise properly. I will leave my equipment in the heat of my van and not worry, but power amps and heads I will bring in, from, say, Nov to March in my area. (Southern Ontario)