Don't waste money on the Creative ZXR, it is a complete joke. It sounds great in games, it really does, but for anything outside of games, the sound is lackluster. Creative also rates it up to 600ohm for the amp..... It cannot power a pair of DT990 Premium 600 ohm, whereas the Phoebus has no problem with that.
As far as coming from the STX/ST, the Phoebus would be a side-grade, not a downgrade. Those that say otherwise are running into issues with their setup, in which something else is more than likely interfering. If it was strictly hardware related to the sound card itself, it would be an issue for everyone, period. The latest drivers from ASUS fixed majority of driver/software issues related to the card.
The only 2 cards I would suggest for games, would be the Phoebus or the Creative X-Fi Titanium HD. Outside of games, a good Claro card or Essence ST/X would be the go-to card.