Perhaps not the "best sound", but it should certainly work nicely. You'd see lot of user shouting that the xonar stx is not good for the hd800, however there are few that made seriously the experience, and then you get the comment "better than expected". I run myself the hd800 out of a xonar stx, and find it's pretty good. I don't know the supposed "mind blowing" experience from a better source (often, interpreted as significantly more expensive) , and I don't care because the hd800 still sounds better than any headphone I tried before.
So here you get an other user like takato14, that would say it sucks without even trying regarding the xonar stx + hd700 combination.
Also, there's jude that tried the hd700 of the audioquest dragonfly, and said it worked nicely
Personally when I see the tiny size of the dragonfly I doubt it would beat the xonar stx (although there's no clear comparisons here).
Edited by extrabigmehdi - 11/22/12 at 5:12pm