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

@decooney I am most often very pleased with my SLP 98. I do listen and critique what I'm hearing with each listening session. I'm still rolling tubes in and out and trying to sort out what I think sounds the best. I have so many possible combinations that I tend to confuse myself. I just picked up some old Shuguang metal base tubes that look very much like the old Melz. They sound surprisingly good. I've been leaving a pair of Sylvania GTB's in the buffer spots for awhile and trying to evaluate all others in the line/gain stage with the same buffers in play. Makes it a bit easier. I had used the EH gold pins in the buffer spot for a good while too. SQ seems about the same as with the Sylvania GTB's. 

You seem to like Psvanes. Which ones? There are quite a few choices. Anyway, not trying to take over the OP's thread, at least he can read about what tubes other guys use and like. 

@rockadanny Didn't mean to steal your thread, just get caught up in the conversations. There is little question that you could make a big difference by picking up a new preamp, just trying to suggest some cheaper options. Tubes vary tremendously. I've been "tuning" my phono preamp with different tubes for about a year and to me, it's just fun to see what a difference tubes can make.

To @billpete - the PSVANE Co on the USA side was contacted by me last year about an older set that had seemed to have lost its luster.  It was determined the tubes were originally made around the time of end of pandemic, with some "known" issues. So, they agreed to replace the pair after me sending photos that I destroyed them [per their policy] with the original box/papers in photos to prove there would be no resale of those tubes. So, these were replaced by the same newer version PSVANE CV181 MKIIs, had something to do with different coating or something, dont recall exactly.  (These guys are are also members on this site, btw) and pop in once in a while. Thew new tubes have been in the front L/R position since.They sound nice for my setup.  

I was not looking for the new ultra-transparent signature we read about in the latest version [other tubes] with different names out this past two years.  I was interested in what they sent [replaced] more of the lush and midrange focused sound these yield vs. uber detail with no midrange body. With my tube systems, I’m not a fan of that type of overly-uber-detail sound. I can get that in my other Class SS amps if I crave that.  I’m using these replacements  in front two [input tube] spots, and have a leftover two sets of TJ Full Music 6SN7s in the rears driver slots. Interestingly enough, PSVANE ended up acquiring all that 6SN7 lineup from TJFM and using that in their lineup now somehow, as I understand it. I enjoy this combo, and end up mixing front/rears some and prefer it.  Here is what I’m using in front now in my Cary SLP-98 preamp. So far so good, they sound nice to my ears, in my setup >> on sale now at PSVANE Co on their site for $199/pair: PSVANE ART TII CV181-TII [Note: your results may vary and you might not like them, fwiw]. Seems to vary by the listener and how the rest of your system and room is set up - i.e. may depend on how neutral/lean -or- weighty and full sounding your room reacts now, so preferences may vary of course. Can't stress that enough, to each his own fwiw.  

@decooney Thanks for the info. I have seen Psvane respond here before, nice to have builders around. I have read a fair amount of complaints about Chinese tubes having a short life. Hope that is not the case with your new ones. It just makes me pause to think before spending $100 to $200 apiece for new Chinese tubes, especially since I know a lot of old tubes that I can buy for less..........so far anyway. I don't rule them out, that's for sure and am encouraged by what I am hearing from the Shuguangs. There are also newer Shuguangs at somewhat reasonable prices but I always worry about counterfeits. At least dealing direct with Psvane, we should be getting what we paid for. Anyway, thanks, much appreciated. 

Upon recently upgrading significantly from an integrated amp (with tube preamp) to a preamp/amplifier, the way I explained it to my audiophile buddies was this.  The upgrade in the external DAC altered the audio image in a high level, clearly performer delineated soundstage; the upgrade in the preamp resulted in more "audio data" in each of those performer positions.  Of course every piece alters the sound signature as well.

First time in the very many years I have been fascinated and participating in the hifi experience that I was able to describe my experience in such a way relative to each piece of equipment.

On the NOS tube search/comparison front, I recommend you reach out to @chickenoregg He’s a nearly limitless font of information and has great inventory. I’ve gotten great 6NS7 and 12SN7s from him.