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

@rockadanny Thanks for the reply. If those Tung Sols are the new Russian versions (which I'm betting they are), then you can do a lot better. They may be OK and sound fine but if I were you, I would absolutely try to roll in some 40's or 50's vintage tubes. Tung Sol black glass VT231 are maybe the most notable but have gotten pretty pricey. VT231 is the military version of 6SN7. I have Ken Rad black glass VT231 and they are some of my favorites. I also have Sylvania and RCA VT231's and all are very good, even excellent. The civilian versions were 6SN7GT and in particular, some of the Sylvanias of this designation are the same as the military version and are excellent, many sold as Bad Boys. The later versions became 6SN7GTA and later GTB and some of these are not bad but SQ has gone down somewhat from the GT's and VT231.s, This would be all old US made tubes. There are good versions from Europe as well but I am less familiar with them. 

I have some Electro Harmonix Goldl Pin 6SN7 (Russian) tubes that probably compare to your Tung Sols. They are not bad and I have no problem using them in the buffer position but they probably compare more with the later GTA and GTB tubes, which is to say OK, not bad but just not great. It is common for tube component makers to include these modern inexpensive tubes but I don't know of anyone who doesn't roll in some vintage tubes to bring their gear to life. 

6SN7 tubes are made in USA, one marked as such,  the other in NJ.

The reason for my post is to find out what more substantial preamps would bring to the table vs. my Mapletree. Hoping an ex-Mapletree user would chime in. Or just anyone may comment. This preamp cost me new < $900, but it seems well done,  and besides,  NOS 6SN7 tubes are used so I expect it to sound good. Just curious what I might be missing,  if anything, with a much more expensive piece. 

I imagine what you're looking for is the naturalness of the 6SN7 with maybe a bit more robustness and elegance than the 2A SE is capable of.  I would look at the Tubes4HiFi SP14.  It employs 4 6SN7s in an "Aikido" topology, which is a very popular noise-cancelling circuit developed by John Broskie of TubeCad.  It maintains the ease of the 6SN7 but offers a richer, deeper tone and blacker background.  It's what I use (in my own DIY version) and I never get tired of it.  You could also look for a used Don Sachs version of the SP14.  He souped them up a bit with premium parts and some modifications and better cosmetics.  A terrific preamp.

@dogearedaudio Nice! Thank you. That is the type of info I hope to find.

@rockadanny 

The SP14 is one of the great bargains in audio, IMO. ;-)  Someone else mentioned the LTA MicroZOTL, which is a headphone/pre combo that uses the 6SN7.  There's also the Icon Audio Pure Valve but it's pretty pricey.

The Cary SLP-98 is a classic 6SN7 preamp.  It's very musical, but I find it slow and thick compared to the SP14, which is faster, cleaner and more transparent.  You can also easily upgrade the SP14 or have someone do it for you.