You should replace any 40 year old electrolytic caps or else move on to newer stuff. Period. As has been mentioned, parts quality in 2020 is much better than what was available 40 years ago, and electrolytic caps don't last anywhere near 40 years.
I will restrict my comments to speakers, since I don't have any actual experience working on electronics. I can tell you that I am currently rebuilding a pair of ADS 1290/2 speaker crossovers. I've been very slow and deliberate about this, replacing one crossover component at a time and evaluating the change after burn in one one crossover while leaving the other crossover stock. I also had the tweeters and mids reconditioned.
As has been mentioned, you can go overboard and change the character of a speaker, which you don't want to do. If you wanted something that sounded different, you would just buy something modern.
At this point, I've replaced about replaced about 2/3 of the caps and resistors in one crossover using good but not great parts. I've left the inductors alone so far. The improvement is not subtle. The crossover with the old parts is quite veiled compared to the partially modified crossover. But the character of the speaker has not really changed. It is quite musical and is seemingly incapable of anything approaching fatiguing.
Having gone this far, I am building a new crossover board that is point to point wired and eliminates the extraneous (as far as I am concerned) switches.
This has been a great learning experience for me and very rewarding. I encourage you to proceed. Just don't be in a big hurry and make sure you have your circuits understood before you start.
I will restrict my comments to speakers, since I don't have any actual experience working on electronics. I can tell you that I am currently rebuilding a pair of ADS 1290/2 speaker crossovers. I've been very slow and deliberate about this, replacing one crossover component at a time and evaluating the change after burn in one one crossover while leaving the other crossover stock. I also had the tweeters and mids reconditioned.
As has been mentioned, you can go overboard and change the character of a speaker, which you don't want to do. If you wanted something that sounded different, you would just buy something modern.
At this point, I've replaced about replaced about 2/3 of the caps and resistors in one crossover using good but not great parts. I've left the inductors alone so far. The improvement is not subtle. The crossover with the old parts is quite veiled compared to the partially modified crossover. But the character of the speaker has not really changed. It is quite musical and is seemingly incapable of anything approaching fatiguing.
Having gone this far, I am building a new crossover board that is point to point wired and eliminates the extraneous (as far as I am concerned) switches.
This has been a great learning experience for me and very rewarding. I encourage you to proceed. Just don't be in a big hurry and make sure you have your circuits understood before you start.