Amplifier caps usually have a life span of @ 20 years give or take. When they start to go, the first obvious sign is "bulging" of the cap. A second sign is a crackly, popcorn sound on high impact sounds like snare drum shots or the like. All caps dry out over time & need to be replaced. Ironically, if a unit is in storage for a number of years, the caps can dry out prematurely from not being charged up..