IMHO, humidity, not heat would be more of a factor in corroding the electronics, however, heat could dry out any rubber parts. Good luck.
Garage Storage?
I recently had several tube and solid state amplifiers as well as receivers passed to me to sell. They've been stored in their boxes in a garage with no cooling for several years in SoCal where it gets pretty hot during the summer. I currently don't have the equipment to test out each of the systems. Assuming they were in working condition before they were stored, is there a big chance that they are damaged from the heat?