The STX already has a very decent DAC using an implementation of the TI Burr-Brown 1792A, which is/was top of the line.
The m-DAC uses the ESS Sabre 9018, which is going to be a bit cooler and more detailed but may sound a bit sterile in comparison.
Another option would be to save half the cost of the m-DAC and go in the opposite direction when it comes to signatures and get the Schiit Modi Multibit, which will sound more natural and musical than a 9018 based DAC.
As you are starting off with a very good DAC, any improvements from buying a more expensive unit will be subtle at best, night and day improvements don't tend to happen above bargain basement audio.