Stefano - Here are some thoughts from having owned, listened to, appreciated and messed around with CS3.5s for decades. You are correct that anything you do will have sonic consequences; and also that some of those consequences will take you closer to the speakers’ original performance. Regarding the tweeters, unless you are hearing problems, I recommend you follow the advice to keep the 28/2s as backups. If you swap, see if Rob at Coherent Source Service can renew your ferro-fluid in your originals to save them as backups. Those VersaTronics caps are high performance, long-life caps. I don’t know of a single failure. However, 40 years is considered their estimated service life, and you are getting close. Some of those electrolytics are in signal paths where their failure would wipe out their driver; so I would replace those for safety. Rob or I can coach you; A’gon disallows sending schematics, etc. Note the 3.5 was the last product with the ultra-bypasses - styrene .015uF around PP 1uF. Great caps, keep them. Also keep your hookup wire.
Caps: better caps exist today and caps are an expensive upgrade. Your biggest bang / buck is to swap the 8uF tweeter feed for a ClarityCap CSA or PUR. These caps will not alter the ’house sound’ whereas other brands will.
Resistors: Jim developed those non-inductive ceramic resistors and they’re better than normal sand-casts. At the time we considered better resistors, but budget prevented their inclusion. I highly recommend swapping at least the series resistors, especially in the midrange and tweeter with Mills MRA-12s. Pretty short money, same circuit performance, sweeter sonics.
Binding posts: If your plastic-cap binding posts work, keep them. They are better than later big, brass posts which were Kathy’s capitulation to market perceptions.
Note: XO values were weaked end of 1987, you want the revision. What are your serial numbers?
Grille frames: This suggestion is just that, offered for general understanding. Those frames cause diffraction, but the fabric was considered in final voicing. IF your room is well damped (soft stuff, especially at wall reflection points), the difference in frequency response is often OK when bare. In that case, best performance is to create a grille frame that functionally fills the baffle edge voids, but eliminates the outer frame members. Conceptually, the new frame would keep the base perimeter and chop off the aerial parts. The long side struts would be rounded over to finish the curve of the baffle. Affix in place with Mortite or BluTac. You improve the anti-diffractive base function and eliminate the diffractive aerial elements.
Equalizer: The equalized bass was fundamental to Jim’s vision - it was abandoned due to market forces. It produces more integrated, better performing bass than the later reflex system. The EQ can be substantially improved (I am close to an available product.) Resistors replaced with metal film, Transistors replaced with lower noise, higher performance, maintainable versions. Caps upgraded as appropriate. Power supply redesigned as regulated rather than present unregulated circuit. Original all-discrete, Star Darlington, direct-coupled design remains. All in, big step up.
There are other hot-rod tweaks which we can discussed via PM if you wish. I posted all this detail for all you who might have been wondering.