I haven't gotten to trying my O2+Odac against the Asus Xonar Essence STX until now. I've been so happy with the Objective product that I've been stuck with it :D
Today I lent it to one of my friends so he could try it with his AKG K271 MKII. So now I only have my Asus Xonar Essence STX and I must say the difference was bigger than I had thought it would be. The sounds isn't as dynamic as with the Objective, and the bass is a bit weaker and "muddier", probably because Xonar STX has a ~10ohm output impedance while Objective has ~0.5ohm.
The Xonar STX is cheaper than the O2+Odac, it supports 192kHz which the Odac doesnt (like that would make any noticable difference?). But still, Objective is the winner. It has superb audio quality for a relatively low cost. I don't think there's any way a commercial product at the same price of the Objective would stand any chance against it. I would even dare say that the Objective combo is the best you can get for <5000SEK (€590) or maybe even more.