So I picked it up today. After a little testing I definitely like it more than ZxR soundwise from a soundstaging point of view but the funny thing is that I still think the onboard Realtek solution on my ASRock Extreme6 Z87 with a ALC1150 chip and two TI NE5532 onboard amps sounds (which is by far the best sounding onboard chip solution I've tried) at least like 80% of that of STX II and has a very similar sound signature too, only small small differences. The ZxR stood out more clearly in sound having a much more bloated subbass response and the highs slightly bit toned down. The STX II in comparison to the ZxR is less bloated in the subbass and a brighter but the onboard may be like ~5% brighter still leading to the STX II sounding ever so bit smoother/more fullbodied than the onboard. For my equipment the STX II and Realtek is a much better fit making the ZxR sound too warm and too bloated in the subbass.
So far it's difficult to say with the stock STX II opamps whether or not I prefer the Realtek or STX II, the Realtek have suprisingly tight and punchy bass characteristics which is highly attractive to me personally, in that aspect it may even be slightly bit favorable but the mids may be like slightly bit favorable towards STX II but I'm unsure whether I find the highs TOO smooth or not. Might need to try some other opamps.
Edit: The other included opamps just plain sucked to my ears, not that much of a different sound signature, just less clear, almost shockingly muffled so avoid those, there's a reason the muse (although cheaper models) are used by default.