Caps in speakers can vary widely in terms of longevity. Film caps and paper in oil caps can last a very long time and some ancient caps are still very much sought after for their sound quality. I Have heard fantastic systems built recently that utilize 80+ year on paper in oil caps. A local builder uses some very old caps that test poorly in one respect (voltage droop) but that is largely irrelevant in a speaker crossover so he uses these poor testing caps because they sound good. Like anything in audio, it is a matter of taste so there are no hard and fast rules.
Don't Neglect to check your Caps!
Lately, my system has been sounding oddly dark.
At first, I wondered whether my recently acquired VooDoo Dynasty ac cords were still settling in, then it occured to me that it might be worthwhile having my 18 year old Silverline monitors checked out.
I found a local tech (Mike at Neal’s Speaker Service in Colfax, CA) who told me the tweeters were fine but the caps were on their way out. I gave him the go ahead to replace them and wow, what a difference! One advantage to waiting as long as I did is that the contrast is dramatic and immediately apparent, not that I’d advocate this as a deliberate strategy.
I guess it’s like the slowly-cooking frog analogy. Our ears can adjust to slowly degrading sound and we won’t necessarily notice it until it reaches a tipping point.