"Circuit board leakage" is rare at the best of times and a complete non issue in a crossover.
It is not a given that caps should be replaced no. Non electrolytic caps don't wear out.... Some speakers don't use electrolytics but most have at least some.
If the speaker sounds fine to you then that's the main thing. Many speakers have been ruined by enthusiastic amateurs messing with them (as have many amps etc!).
It needs to be understood that good speaker designers will have taken account of the shortcomings of the cheaper parts used in the crossovers of often even expensive speakers. Electrolytics have very wide tolerance margins, +/- 20% is good for electrolytics! They also have quite high ESR (equivalent series resistance), as do more affordable inductors in most speakers. Replacing either with ostensibly much better quality components can have a very bad effect on the flatness of the frequency response of the speaker.
Overall the only components likely to wear are electrolytics. To test these you need to be able to measure the capacitance, and the ESR. ESR meters are rather less readily available than a multimeter with a capacitance range but can be obtained these days. Measuring just the capacitance is usually pretty useless...
Now when you do measure the capacitance and find that 22uF cap reads 17uF do you assume it's faulty? That's actually within spec for one thing and for another, how certain are you that the manufacturer was not selecting ones around this value deliberately? Some do. Such as KEF in all their Reference series speakers. Some will not be too picky and are happy to just fit 22uF caps knowing fine well they could be 20% higher or lower than that.
To get to the point, I advise leaving everything alone except, in the case of very old speakers, electrolytic capacitors and then they should be replaced by a very similar equivalent part, not one of vastly greater quality.
If you are planning on using polyprop caps from the likes of Mundorf etc and flat tape wound air cored inductors then you should know what you are doing to a degree where you certainly would not need to ask the likes of me about it! In effect, you would need to be capable of redesigning the crossover to allow for the superior characteristics of these components.
Arkless Electronics-Engineered to be better. Tel. 01670 530674 (after 1pm)
Modded Thorens TD150, Audio Technica AT-1005 MkII, Technics EPC-300MC, Arkless Hybrid MC phono stage, Arkless passive pre, Arkless 50WPC Class A SS power amp, (or) Arkless modded Leak Stereo 20, Modded Kef Reference 105/3's
ReVox PR99, Studer B62, Ferrograph Series 7, Tandberg TCD440, Hitachi FT-5500MkI, also FT-5500MkII
Digital: Yamaha CDR-HD1500 (Digital Swiss army knife-CD recorder, player, hard drive, DAC and ADC in one), PC files via 24/96 sound card and SPDIF, modded Philips CD850, modded Philips CD104, modded DPA Little Bit DAC. Sennheiser HD580 cans with Arkless Headphone amp.
Cables- free interconnects that come with CD players, mains leads from B&Q, dead kettles etc, extension leads from Tesco