I finally took some time to perform this test:
- Foobar (KS) -> STX -> M3 -> AD2000
- Foobar (KS) -> DACPort -> M3 -> AD2000
Listening was done on a ATH-AD2000, FLAC files, played over Foobar2000 via kernel streaming on both interfaces. No resampling, no filtering, STX driver set in Hifi mode at max volume.
Comparing both, the transparency is clearly nicer. Mids are more emphasized, sharp, full, creating a more involving listening experience. PRaT is also nicer on the DACPort.
STX sounds a bit laid back in comparison, not too much though, just a bit vs the DACPort.
One aspect is, the bass extention and impact is clearly better on the STX. The low bass is fuller, with more energy, more kick. Note the difference is thinner than on the previous listening where I was using - Foobar (KS) -> DACPort -> AD2000.
Interestingly as well, despite a better spatial positioning and instrument separation on the DACPort, I perceive a wider soundstage with the STX, the sound is simply "wider". DACPort is more "in your face".
Edit: I like the bass guitar on the STX better. More realistic.