Filter capacitors have a half life of about 20 years. What that means is that in a 20 year period, about half of the caps will have failed. This is true of transistor and tube amps.
Sometimes when the cap fails the ESR goes up like Kijanki was talking about, you can also have the experience of the cap slowly loosing its capacitance. This may not result in the hum that is commonly associated with filter cap failure, but it will impose sonic degradation as the power supply is no longer properly bypassed. This can increase distortion and affect bandwidth, things that you may not notice over time but would certainly show up on the bench.
So in general, by the time a piece is 15 years old, there will be some degradation or outright failure, by the time 20 years has passed the chances of that are 50/50. This will be true even if the unit has been in continuous use, although such pieces in general do have the caps last longer, that's not the same as saying they actually still meet spec!
Sometimes when the cap fails the ESR goes up like Kijanki was talking about, you can also have the experience of the cap slowly loosing its capacitance. This may not result in the hum that is commonly associated with filter cap failure, but it will impose sonic degradation as the power supply is no longer properly bypassed. This can increase distortion and affect bandwidth, things that you may not notice over time but would certainly show up on the bench.
So in general, by the time a piece is 15 years old, there will be some degradation or outright failure, by the time 20 years has passed the chances of that are 50/50. This will be true even if the unit has been in continuous use, although such pieces in general do have the caps last longer, that's not the same as saying they actually still meet spec!