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

@dogearedaudio Are you talking about the side getter GE's? I have quite a few and they are probably my favorite of the GTA and GTB types. I have them under several names but all are the same and are tied to GE/Ken Rad, from a few different factories. Some are "O" getter and some are "D" getter but I'm not sure if one sounds better than the other. I agree with you on the Sylvania Bad Boy types and I have them in both the VT military version and the GT version. Any of these bad boy types are probably my overall favorite 6SN7 but the RCA smoke glass GT's and VT231'a are nice too as are the Ken Rad black glass VT231.

The only ones I"ve tried with adaptors were 7N7 loctals. I didn't see any advantage to them but I may try some 6F8G types at some point. I might also check into the 6J5's as well. I enjoy the fiddling around with different tubes. It's kind of like "tuning" the preamp.

@rockadanny Too bad about Brent retiring but good for him, I guess. There are plenty of tube suppliers and I get most of mine from Ebay. You just have to be careful, check feedback, I don't like to see any complaints but you have to be able to read between the lines too. Some people are impossible to please  but more than 1% negative FB is probably well deserved. I try to deal with 100% positive FB whenever possible. I've had good luck with Miller tubes and plenty of other smaller sellers.

@decooney There are two large gold colored Jensen caps in mine and I know I was told that it had oil caps as well as being direct coupled, having Hexfred rectifiers and mine also is a 98P with Lundahl SUT upgrade. Beyond that, I really don't know if there is anything else besides what made it an F1 in the first place. At some point, I'll probably send it to Cary for a "check up" and see if it would benefit from any repairs or upgrades. I know the Mundorf caps get mentioned and @harpo75  talked about some improvement to the PS as well. He worked on a lot of these and designed a good part of the SLP 05. Haven't seen him in awhile, I hope he's OK. 

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.