Here's how I hacked my Essence ST - I believe the STX is the same DAC hardware, just a different bus interface.
The ICs ZU14- ZU16 need to be rolled - fortunately these are socketed (btw these are beneath the snazzy screen, so that should be removed first). ZU14,16 are exchanged for LM6172 and ZU15 becomes an NE5532. I must admit I haven't tried other options for ZU15 just that I found 5532 sounded better than the supplied 4562. ZU14 & 16 though definitely sound best with LM6172 out of a number of options I tried.
The other part of the mod requires some soldering skills. The caps adjacent to pins1 and 7 of ZU14 &16 need changing for 10pF, 0805 footprint. That's 4 caps in total to swap over from the existing ones (which I believe to be 2200pF though I destroyed mine in the desoldering process so wasn't able to check this).
Subjectively this mod gives greater depth to the soundstage and seems to resolve lower-level details better. Total cost was about $3, for the money its excellent value