What do bad caps sound like?

My MF Nuvista M3 is out being re-capped at the moment - preemptively as I don't think I had an issue - but just out of curiosity, if caps were to go bad in an amp is there a particular sound that makes, or specific degredation in sound quality?
Bad caps usually either explode (rare) or leak (common) or just stop working. (usual story, with some leaking)
Twenty years is the general rule for big caps in amps. Best to replace them at twenty. Small caps hard to say. some die fast. (IMO the most common fault in electronics are small caps going bad, and taking out something important) some last forever.
The twenty year rule can be used.
If you replace all the caps in a product, you get a like new product good for another twenty.
When they went out in my subwoofer amp they made the driver just go pfft pffft blffft pfft pfft. Pppppppb pppppb. BTW- I'm not kidding. Really. REALLY! You should have heard me trying to explain it to Albert at VSA.

You could also SEE that the caps had gone out as the tops were flat but when they went out they were domed.
You will usually get a hum that may increase while your amp is playing music and the caps discharge. Bad power supply caps can no longer block ripple current.

Depends where they are and how "bad" they are. Jallen
Rwwear- Great link. Really interesting. :-)