I just remembered another "failure"...

Several years ago I bought a KLH Audio (sadly, now out of business) powered
woofer, which I tossed in the garage. A week ago I finally got around to
pulling it out and connecting to the LFE output of my BD player. (This was
intended mostly to keep the main speakers from being overdriven.)

When I turned it on and up its gain (that's "syllepsis"), it made "pumping"
noises, all on its own. After a few minutes of "charging up", this went
away. It would seem that after a few years of non-use, one or more bypass
caps had deformed, allowing instability.

I used to own several KLH Model Eight radios, and though they were 45 years
old, none needed a capacitor replacement.