"If it ain't broke don't fix it." I own and have owned many pieces of vintage equipment 40+ years old that I start on a variac and they sound terrific without ever having any maintenance besides tubes.
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If youre not blowing fuses and all seems well, all probably is well. But if youre itching to do something, I know people who swear by the improvement they realize by regularly cleaning and polishing contacts (PC, IC and tube). FYI, cj offers a renew" service for their older models, but unless you feel that somethings wrong, Id wait another ten years before I explored that option. Ive always been happy with cjs advise when Ive called.
Other than periodically replacing the tubes (and they generally last longer than most people think), I would just occasionally treat all of the contacts with Caig Deoxit or Cramolin. You don't need much of it at all, but I think it's a good idea. For a quick treatment, just spray a tiny amount of deoxit into the end of your interconnects and then plug it in and rotate the end to spread the love.
I'd also advise testing your tubes before replacing them. Don't replace based upon some arbitrary limit. If something starts to sound wrong or you blow a fuse, test all the tubes. You can probably find an old Eico dynamic tube tester on eBay for about $200 or less.
The only other thing I could think of is just opening it up and making sure to keep it clean/dust free inside.
Generally speaking, if something is wrong you will most likely be able to hear it. The problem with proactive maintenance (especially with vintage gear) is that you can change the tonal quality - and not necessarily for the better.
Contact cleaning is fine. I would advise against changing/replacing any internal parts other than tubes if the preamp is working fine.
By the way, I mentioned Cramolin but haven't used it for at least a decade. Cramolin violates some EPA regulations and was not rated as safe for plastics. Cramolin is no longer readily available, and I only mentioned it in case you happen to have some lying around. The Caig Deoxit Gold 5% solution/Pro Gold are much better. Interestingly, you cannot legally ship Deoxit/Pro Gold through the air - it must travel by ground. And if you order more than one can, it must travel in a separate package. I think this only applies to the aerosol version.
Anyway, the point is if you already have a can of Cramolin lying around, go ahead and use it, but sparingly, wiping off any excess with a lint-free cloth. Since you probably don't have it, order a can of Deoxit Gold 5%. You can find it any number of places online if it's not carried in your area. Musician's Friend had a good deal on it recently, and you can definitely find it on Amazon.
I have used a CJ premier 14 for fourteen years with no maintenance other than tube replacement every two to four years. The only contact cleaning is with a rough cloth or br turning the RCA connectors a few turns. Sounds as good as new. Ed in the service dept can be very helpful. Thet are this week, but try them in the new year.