Particularly tube amps degrade very gradually, so that it is difficult for the user, familiar with the sound, to determine when this degradation becomes serious. Objective measurements, which the manufacturer is best qualified to do, are therefore appropriate.
Transistorized amps, on the other hand, usually go "thump!" and blow a fuse. Hard to miss.
Transistorized amps, on the other hand, usually go "thump!" and blow a fuse. Hard to miss.