Depending on which amp, i.e. the larger number of output tubes used, the hotter the amp runs. I have the MA-2's which consists of twenty 12AS7's per mono-block. Hot, at times, is an understatement. I live in Wisconsin, so a good portion of the year temperature is a non-factor. The only other A-S amps I have heard are the MA-1's. Fewer tubes, less power, less heat.
Regarding tube replacement, the drive tubes, 6SN7's have a reputation as long running, consistent sounding and reliable; especially NOS. The outputs....not so long, but not all that expensive.
My amps, nor the other two I have personal experience with have ever had reliability issues. Do take Scifi's suggestion and research either ASOG or threads on Audio Asylum.
The amps are very impressive. Well worth investigating.