I am not certain that I am partial to the Sennheiser sound, it is the only pair of decent quality headphones I have owned. I do think they sound good, but don't have much to compare them to. They are a hand-me-down from my father who used them for his studio recording stuff, they had some major damage to the plastic that holds the adjustment band, but performance wasn't affected, it was merely cosmetic.
I have been looking for a better solution, but I am not sure about the open headphone design, the passive noise canceling the HD280s have is probably essential to my listening experience, but that said, I don't have much experience with other solutions.
I would hate to ditch the Xonar Essence STX card if I can avoid it, I already did spend quite a bit of coin on it, and thus far I am quite impressed with it. I have used Creative (Crap-vative) cards and M-Audio, among other cards previously. It is surely in a class far beyond. As for the jitter -- I have heard it can be an issue if you are up-sampling a signal rather than running it at a native rate but I tend to run native. The solution you suggested also is not capable of 24/192.