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

Brent’s retired. :-(  12SN7s are fun, a bit "lighter" weight in sound due the lower cathode current.  My go-to 6/12SN7 is the GE GTA or GTB.  They are plentiful, affordable, can easily be found NIB.  If you haven’t tried them you should.  They have a full, textured, smooth sound.  I use them in my amps and preamp. ;-)  Another good 12SN7 is the Sylvania "Bad Boy" with tall, rectangular plates--facing each other, not angled.  (I don’t care for those at all.)  And if you *really* want to try something different and interesting, get some adapters from eBay and use 2 6/12J5’s in place of each 6/12SN7.  It’s a nice sound!

"@billpete ...Without good tubes, a very solid piece of gear can sound pretty dismal. I think you would agree with that and is pretty much the point that I’m trying to make to the OP." 

Sure, Yes, Agree. 

And, fwiw, what I figured out over time with my former Cary SLI-80 amp, current Cary SLP-98 Linestage, and former Cary V12R amp is this: 

• The tubes make a difference, yes.  What made an even greater difference was changing the caps, adding better diodes/resistor parts, (standard upgrades) inside, and THEN all the sudden any tube swap/changes and comparisons made an even greater and more noticeable difference.  
• When my buddy removed his older Jensen oilers and replaced them on the SLP-98 (older version) with the new Mundorf  silver-gold-EVO caps. He called me up kinda stunned at the resolution change. 
  Get this, then he ordered an all-new 2nd version SLP-98 with all the upgrades in it, and sold the older 98 to his buddy to have.     
• Some refer back to the decades old story of the "bad batch of Jensen oilers" with those caps, and all I can share is the newer/better Mundorfs worked differently, much more clear and transparent with higher resolution, if that’s what you prefer.  Worth checking the caps too and trying the swap if anyone has the old thick sound still etc.  

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.