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

@decooney I know that you have a lot of experience with these preamps and the tubes that they use. Mine has the Hexfreds and is direct coupled, which I"m told, increses the speed. I don't understand half of it, just know what I'm told and how I perceive the way it sounds to me. I think of it as sounding mostly very natural with great detail, excellent treble and mids and solid bass with good slam, all of which is completely dependent on a very good tube compliment. 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. 

Thanks! I'll seek out some better tubes, tho my main source,  Brent Jesse, is closing (has closed?), from what I've read somewhere. 

I can run 12SN7 with the flip of the switch. Any 12SN7 as good or better?

@rockadanny I don't know the 12SN7's as I can't use them but they are often cheaper since not many can use them. Ask Brent. He'll know what to do and will have something that matches well with your gear. 

March 2 was final day to order tubes from him. Super bummed.

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!