When is used used up?

Al - Do you remember to increase line voltage slowly over few days? Voltage re-builds up isolating layer of aluminum oxide in electrolytic caps, that tends to get thinner without voltage lowering breakdown voltage.

"Capacitors do NOT DEGRADE over time, they FAIL."

- I don't know where you got this from, but even capacitor manufacturers admit that electrolytic capacitors dry out (and rate them). At room temperature they will last for 30-50 years but temperature accelerates process greatly (each 10degC cuts life by 50%). ESR of capacitor increases over time and, in presence of big currents, causes internal self heating (just few degrees) and so forth.

Power supply capacitor is in series with a speaker (circuit closes thru power supply) and any increase in ESR will show as worse bass control (lower DF) and loss of dynamics. We get slowly used to "new sound" but difference after replacement can be huge.

There are electrolytic caps that are better quality, lower ESR, lower inductance (like slit foil caps) and higher temperature ratings but they are expensive. Better (high end)manufacturers most likely use better parts.

Norman - Not likely to find White Papers on basic stuff like that.

Amp's output current flows from power supply capacitor thru it's Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL) then thru output transistors and speaker to come back to power supply capacitor.

Amplifier attempts to regulate/mantain required voltage level but power supply cap's ESL will make it less than perfect at higher frequencies while it's ESR if high enough will cause high voltage drops and eventually amp will run short of voltage.

ESR of capacitor is constantly increasing because of drying effect up to point that after long time self heating (big current x ESR) will cause thermal runaway effect (ESR increases with temperature) and capacitor might even explode. To prevent catastrophic explosion all larger electrolytic caps have built-in fuse (rubber plug).

Quality of sound depends on quality of power supply caps (ESL, ESR, leakage etc). Best caps have extremely low ESR and ESL. Sometimes people bypass electrolytic caps with much lower inductance film caps (to speed-up response) but this creates parallel resonance circuit with ESL (and therefore ringing).