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

@dogearedaudio It is the Cary SLP 98 preamp that I base all my 6SN7 opinions on, although mine is the F1 version, sort of a hot-rodded version made for sale through Upscale Audio. Anyway, I don't find it to be slow or syrupy or thick or whatever other descriptions have been used on the original SLP 98's. 

@rockadanny It's good that your Tung Sols are US made but they are not the premium Tung Sols by any stretch. Buy a new preamp any time you want but if it was me, I'd sure try some better tubes first. It can make a huge difference in how a preamp sounds. Worth a try if you ask me but it's certainly your call to make. Keep us posted on what you do or what you find. 

About the Cary SLP-98 Linestage/Preamp topic and variations out there today. 

Yes, some of the older "F1" custom versions out by Upscale Audio and others that copied that sounded "thick".  A lot changed after that and Jensen oil cap leakages. Then came newer versions, caps, and varying tubes people use(d). 

Comparions vary greatly: 

A good friend had the old and brand new version SLP-98 and tried it on four amps at his place, and there was a LOT of difference from his old version vs new version. He had multiple SS and tube amps there, including my big Cary V12R amp too. 

There have been a least 3 versions of them before and after Dennis' departure from Cary.  Between myself and three other friends, we've owned four in total, and each sounded different.  The old version SLP98 and newer versions with the Top Shelf Mundorf EVO silver gold caps sound quite different, way differeint imo.  

Variation to the sound -

Caps, variation: 

-first dictated by the internal Caps installed stock (or options Jensen oilers), then later came (MIT, Auricap, or Mundorf to improve clarity and transparency) you'll see out there.  

Tubes, variation: 

I've had several different pairs of 6SN7s and can share this. All Different soudning.  Factory Electro Harmonix then vintage 50s/60s RCA, Sylvania, later TungSol, and modern era TJ Full Music, PSVANE two different generations installed in my SLP-98. None of these tube pairs front and rear sound the same in the 98s I heard. 

Hexfred Diodes, y/n, variation:

-Some had these installed as an option, known for fast switching times, which help reduce noise and improve the "speed" and clarity of the musical presentation.

Switch, Volume, options::

Grayhill switch option and Alps volume control

Some older notes refer to as V1 and V2 versions of the same preamp too, many years back 7+ years ago.   

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Way too many variables, options, tubes -  to characterize one sound of the Cary SLP-98 linestage / preamp.  

That's all good info about the Cary.  I have an older version with the large generic oil caps.  It's not BAD, but compared to the Aikido circuit, it's slow and thick-sounding.

@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?