Hard to describe but if you hear a sound just like tiny/tight fart when you turn the amplifier off, that's usually a good indication that a recap might be required. My real old Adcom 545 did that and my 555 has started doing too. From the sound quality perspective, both amplifiers have much softer bass now as compared to when they were new which makes the overall sound rather flat and lifeless. Both have been used (actually abused) on a daily basis since1987 by me and friends who (still) borrow the 555 for parties.
Recapping your amp
I was wondering how you would know its time for a recap on your amp. Even when it sounds fine and the caps are not bulging or leaking.