Aging of capacitors within crossover networks

HIGHLY unlikely. I suppose if some component is REALLY low quality, its value might change slightly over time, but otherwise, no.

The dialectric material (plastic film, usually) in capacitors will change sound quality slightly as it 'takes a set' due to electical voltage passing thru it (generally called breaking in), but that's all.

BTW, the entire issue of 'passive'-component values in crossovers is highly overrated IMO. Understand that most caps, resistors, and inductors in crossovers are of a + or -10% tolerance from the beginning, and some are + or - 20%! Also understand that it takes doubling or having of a component's value to change its effect by one octave.

However, if you're after highest quality, it makes sense to upgrade the passive parts in a highly loved speaker. Doesn't matter is it's a month or 2 decades old, it's HIGHLY unlikely the manufacturer installed the best-quality parts when he built it. Increasing the quality of parts in a complex crossover could easily change the retail price by $100 - $500 each. For instance, the crossover in my Aerial CC3B contains 6 film caps (plus a 150uF electolytic), 3 of which are rather large at 33uF. All the midrange- and tweeter-series caps were Axon 'propylenes without any bypasses. There are several better-sounding but still affordable caps than Axons, and my the sound from my CC3B benefits from some of them.

So replace because they're old? No. Replace with higher-quality parts for better sound? YES!
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Coupling capacitors in tube amps are famous for degradation, but this is attributed to the high temperature environment and the high dc voltage they see. Film capacitors in a crossover network will probably last forever.

"'passive'-component values in crossovers is highly overrated IMO"

Disagree. Small changes here can make huge changes in sound.

"caps, resistors, and inductors in crossovers are of a + or -10% tolerance from the beginning, and some are + or - 20%"

You don't think the person building the crossover tests them all? Come on.

"Also understand that it takes doubling or having of a component's value to change its effect by one octave."

that depends on the crossover topology, not true in all cases, AFAIk.

"Replace with higher-quality parts for better sound? "

Maybe, maybe not. doing so presumes you know more about the parts/sound/speaker/crossover than the designer, which I find is not often the case. Parts usually were chosen for a reason, and that is not always price.

"Axon 'propylenes without any bypasses" - which is not a bad thing. Axon (aka Solen) is basically about the best film cap that can be had in larger values (say, over 10uf.) in the .1-1uf range there are lots of choices however.

Regarding caps degrading - its' mostly electrolytic types that have this problem, and then usually in stuff that runs higher voltage than speakers. (e.g. amps, power supply bypass, etc.) IN that case, replacing elytics is a good idea. Even better is getting rid of them entirely and replacing with oil or film caps.

-Ed