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You could always send one to GR Research.
Of course it's hard to be 100% certain, but I'd bet all those components are still doing exactly what they are supposed to do. Put on some good music and enjoy:)
You could always send one to GR Research. They will measure its anechoic response and either recommend better parts or design a new crossover to address measured issues. It's not likely that the crossover design needs reworking for fundamental design issues, but they just got a Polk speaker and posted the results on their YouTube channel: if the crossover is solid, they suggest upgraded parts and post the kit for others who want a one-stop DIY upgrade as well.They don't listen to the results, they just go by the measurement of the one speaker and their own experience. They don't charge for evaluating the speaker, and I don't know if they make a significant amount from markup on the parts. They claim they do not.They do not charge to evaluate the speaker because they can post the results for other owners to buy parts kits. So, I suppose if you are in doubt about their recommendations, it's just a loss of time and shipping (which I assume they ask folks to cover).Obviously, they have a forum here where you could get more info if no Sapphire owners come forward with DIY upgrade experiences. But 30 years is beyond the typical lifetime for a capacitor to remain in optimal working condition.
IMHO: This is a fine approach if you have a pair of speakers that you like for a lot of reasons, but for whatever reasons, they sound bad to you. However, IF the speakers were designed well in the first place, there was a LOT more to it than an FR or two. In our process, we made many measurements on axis, off axis (above and below as well as laterally many angles). We also measured and took into account many other factors including frequency-time info and distortion. And we tried to correlate the measurements to listening tests. IF you change caps, you definitely want them to better matched than the standard 10%, we matched left and right much closer than that. And, you want them to be as close to the same actual value as the original design, or it will certainly sound different, but highly unlikely to be better. Main thing though, have fun and enjoy the music, so if you want to switch out parts, try to keep the values as close as possible to the originals both in their primary value (IE: inductors= inductance, but DCR is also a factor).
If you are curious about upgrades on the DIY amp side of the equation, the new Hypex Nilai kits objectively sound much better than the older ICE modules. Alpha Sound has some very well-controlled listening tests comparing the new Hypex to ICE and Purifi Class D: https://www.youtube.com/watch?v=NdZFAWe1NxoWith reasonably good headphones, the differences are easily audible. Their recording studio, microphones, and reference system are excellent. This is not some guy with his phone recording audio while wandering up to the speakers to get a closer look at the woodgrain.The kits are available here (though I understand they go out of stock): https://www.diyclassd.com/products/diy-amplifier-kits/