Do amplifiers deteriorate if not used?

SMPS = switcher (switching mode power supply).

Follow Gs5556 advice and use Variac. It might not help if you have SMPS (not likely) but will save your amp otherwise.

Yioryos - I just assume your ML27.5 has linear power supply. If it has SMPS it will not do anything (start) to about 70VAC then it will put full operating voltage on the caps.

Electrolytic caps last at least 15 years - even more if temperature is low. They dry-out so keep storage temperature low. It is possible (and often done) to recap whole amplifier if necessary. Other components don't deteriorate.

Gs5556 - I suspected it has something to do with overvoltage. I read somwhere about hydrogen release (and explosion) in dry aluminium electrolytic caps.

On the other hand datasheet of the few brands I checked show shelf life only 1/2 of load life and my company uses sometimes in production (special orders) very old caps stored for few years and everything is fine. They shouldn't but we don't have system to trace "perishables".

Could the difference be caused by extreme working condition in many high power amps like high ripple current or voltage close to rated for the cap?