When should caps be replaced?

Thanks for the response.

Hi All

Newbie here. I read the thread and feel that my old 3240PE and 3020 A and 3020 B neither give the symptoms described. To my ear (which is very subjective I suppose :) ) they still sound sharp, clear and base is what I’m used to. So for my equipment, I feel leave well alone. You agree?

@ ahidler: my 1978 Son of Ampzilla has been in daily use (always on - never off) for the past 2+ years! Still sounds excellent! So your vintage NAD 3020 and 3240 are probably ok too! It's anybody's guess how long caps operated within their voltage and temperature rating will last! BTW, I used to own two 3020's long ago - great sounding integrated amps!

The Son has been playing music for 16 hours every day with no problems!