Phoebus does have Dolby Home Theatre v4, rather than Dolby Headphone found on the majority of Xonar cards. No experience of it myself, and there are very few comparisons of the two that I can find. According to Dolby, DHT v4 uses the same algorithms for headphone use as Dolby Headphone. Can't see much point in buying a Phoebus, if positional sound isn't going to be any different compared to the D2X.
Buying a Soundblaster Z would make more sense as it uses Creative's own system rather than Dolby; unless of course, you want to avoid Creative at all costs.