I used to have a SB Touch as my digital front end. It's a great piece, and I still have it, but in a secondary system. At one point I took the digital out and fed a Metrum Octave ($1k) about 2.5 years ago, and it was a significant sound improvement - as it should, as I added $1k to a $250 unit!
Later I replaced the SBT with an ancient Win XP laptop playing JRiver and an Audiophilleo with PurePower (another $1k), and the sound again was very noticeably better. This was 1.5 to 2 years ago.
Early this year I built my custom server, similar to the most powerful CAPS v3, but with a server board, etc, using Windows Server 2012 and AudioPhil's Optimizer and the sound improvement was super noticeable.
Over the past two years digital has evolved a lot and we now see DACs under $1k that have features we couldn't hope for only 2 years ago.
I'm in the process of getting two new DACs: I already received an exaSound e22 ($3.5k) and I'm getting an iFi Micro iDSD ($500). I wish I had the Micro iDSD with me so I could answer if it beats the SBT and by how much. It has gotten a lot of praise, though. And has volume control.
DSD is something to consider even if you don't plan to buy your albums again yet in a different digital format - I know I am not. But there seems to be something about upconverting regular 16/44 files to DSD at the server and feeding that to DSD capable DACs.
When playing 16/44 and even 24/192 files natively and feeding the DACs, the Audiophilleo+Metrum sound a little better than the exaSound (so far, after 2 weeks, in my system, etc). But when upconverting to 2xDSD on JRiver and feeding that to the e22 it does sound A LITTLE better than the AP+Metrum. A little means it's not worth the price difference in my book, but then JRiver is not best regarded for its upconversion ability, so I'm still exploring. Many claim converting to DSD makes a very significant difference.
Sorry for the long post. Bottom end: at $3.5k you definitely surpass the SBT by a very wide margin. The exaSounds are designed so the hardware upstream doesn't affect them and they have a good hardware volume control. But there are many more DACs you should look at too in that price range: Auralic Vega, Chord's Qute and Hugo, etc.
from my readings, my guess is for $500 the iFi is likely to surpass the SBT. But it is a guess so far.
Playback software: within Windows, JRiver has the best interface and sounds good. If doing DSD, look into HQPlayer (I'll be doing that today!).
Consider your server as part of your digital playback chain budget, and also the USB cable. And also linear power supplies for the DAC and server. Some DACs are designed to isolate themselves from hardware-induced jitter so it doesn't matter much. As an example, the e22 is expensive but you wouldn't need to spend much in a server or its linear power supply, or the USB cable. And that would be more expensive than a $500 DAC.
Yet another route is to get a VERY good used DAC. One option could be the PS Audio PWD, which has seen its used-price significantly drop as PS has introduced the new DSD-capable unit.
I hope it helps.