What improvements would I get with a "better" preamp?

Hey billpete.  I’m doing well, I try to stay out of some of this stuff.  You can probably understand why.  
Unless you’re having an issue with yours or you notice oil leaking from the oil cap I wouldn’t worry about a check up.  Everyone at Cary is new now so I really don’t know what they’re doing anymore or how far they take upgrades.  The other tech (Marc) and I came from a long tube background and would really think about how we could get the best sound quality from each piece. I modified my first Dynaco Pas preamp and Mark IV amps and Altec amps back in the early 1980’s and was hooked on tubes ever since.  Along the way I learned more about solid state and digital working with Mike Moffat. When he started Theta Digital I went to work for him way, way back.  I was in and out of audio through the years but it’s always been my first love. Many years of fun! 
I still work on some equipment here and there.  I recently repaired, upgraded and retubed a fellows CAD-805’s for him.  I forgot how heavy those were!
 

@harpo75 I am so very glad to hear from you. Everything in my SLP 98 and it's PS, looks absolutely pristeen.........so maybe there is nothing to worry about. I don't understand when people refer to this preamp as sounding thick. I don't feel that way but maybe I don't understand the term? Anyway, great to hear from you. I may reach out to you with Cary questions from time to time if that's OK. As to getting any of these things worked on, I really have no idea where to turn and hesitate to even involve Cary for the reasons you have relayed. 

From what I understand they have gotten some good technical people.  I just don’t know what backgrounds they have in the tube world.  Please don’t  get me wrong. It’s just that the techs keep changing there.  The young guys don’t stay long like we used to. 
I don’t mind at all answering questions.  I even take on a job here and there.  I’m just careful how much I do now days so I don’t get too sore.  You know, when you’re retired a few bucks once in a while sure doesn’t hurt. 

Hey @harpo75 good to know about you.  Marc and I followed along with each other and after he left and into tube Mic gear and stuff.  We stay in touch on FB some, with similar hobbies growing up.  He helped me in my early SLI-80 and V12R upgrades and right in to the SLP-98. I connected Marc with a local tech nearby who did some amazing upgrades to my SLI-80, and Cary ended up buying that amp back from me full price, they wanted to see it, hear it, and had it in the showroom a while. A customer really wanted it.  I wish I had not resold that one. 

@billpete following along with harpo75, as long as the caps dont appear to leaking and not bulging or anything, maybe you have some of the good batch caps.  if you like the way it sounds now, could be a non-issue and happy listening.

IF you do have a chance to have someone check it or curious to have it upgraded, I can share with you my buddy’s F1 got upgraded at first (caps only) and he really liked the cap upgrades since he wanted even more transparency. He had Harbeth 40.1s, and the midrange did sound a little thick [with his speakers, as I understand it] so that is why he went with different caps in that older F1 style unit.  He eventually ordered another all new 98 with ALL of the latest upgrades. Each case might be unique to each persons gear, room, hearing, and listening preferences.  The downside of that cap change is you might lose what you like now if the current tone, character, body, level of detail, and presentation suits you just fine as-is.

Might only be worth doing any of those upgrades IF you are looking to change the sound in a particular way, wanting more transparency or opening up the sound stage more. One of my friends changed caps obsessively, 5-6 times, and eventually ended up going back to the original caps liking it better. Worth thinking about more. Go with what sounds best to you, all that matters :) 

Decooney is right on target speaking to the areas of the sound quality differences that capacitors can make being a positive or negative in your system.  If the sound fits your system, as in not being too bright or harsh or on the other side, not being too veiled and dark, then you may want to stick with what you have.  If you like the sound you have but are looking for more air and “hear through” into the music then you may want to look for a higher quality capacitor or you may just want to improve your power supply filtering.  My system got so detailed at one point I had to make some changes to make it fuller and weightier.  
One thing I never had anybody dislike is using large value bypass caps across all the large screw top electrolytic PS capacitors.  Of course, we typically used the Solen capacitors because you could get large values of polypropylene capacitance in a small package at a low price.  You can then use a smaller value, very high-quality cap across the solen capacitor.  It really cleans up the power supply and improves the overall sound quality giving you much more transparency and sound stage. A blacker background.

Also, don’t forget acoustic room correction!  That makes a huge difference.  Even just small things as in a few acoustic tiles or a few pieces of foam in the right places can really help. 

I built a completely hard wired version of a SLP-05 with a very large external power supply that has a larger transformer, choke input and several 100uf Solen capacitors in it along with much larger electrolytic cans. It also has some other tricked out dual mono regulation circuitry that divides the line sections from the balanced sections.  I really enjoy being able to sit down to listen music. Every time I bring someone in to listen they turn around and look at me and say something  like, this is more like an experience than it is listening to music.  I say exactly!