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

I second the comment by audio mirror on the overall impact of a preamp upgrade.  I just upgraded from a Rogue Audio RP-7 v1 to a Rogue RP-9 v2, and the results are simply astounding.  The move up to the RP-9 included a move from 12AU7 tubes to the 6N30P tubes, and the move to the version two (V2) included higher grade parts.  The result was a major improvement in lowering the noise floor, greatly improved bass response, better definition and location of sound sources, enhanced clarity and tonal quality of instruments, and modest improvement in sound stage, which was already quite good with the RP-7 in my system.  The benefits of upgrading the preamp were FAR beyond what I anticipated.  The Rogue people have told me there will further improvement as the unit burns in at about 100 to150 hours.  This upgrade has brought me closer to the “they’re in the room” experience.  I suspect that almost any true upgrade will bring some of the same benefits.

I know when I bit the bullet to buy my McIntosh C2800 my experience improved greatly. Plus I now have dual sub outs and a plethora of inputs. Im a believer in spending the money on a great 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.