I think you have chosen a good setup. I personally prefer the DT880s to the Q701s, but they are very similar headphones. I believe the 880s will have more bass when attached to a meaty amp, although the Q701s will have a more distant sound stage, which helps with positioning.
I have used them with a Xonar D2X, and assuming the DAC component is the same between the D2X and STX, I can certainly say that soundstaging and positional audio are top notch with that card. Listening to my DT880s jacked straight in to the D2X gives better positional audio than my $500 desktop DAC+Amp (of course it's unamped so it sounds worse in all other ways
Also, with the Asus drivers, you have access to the Dolby Headphone DSP. If you're playing a game which supports multi-channel sound, and you are set to 8 channels in the Asus Software (important!), the result is quite amazing. For the price of adding a bit of reverb, you get a holographic soundstage that you feel like you can reach out and touch. Very handy in FPS games!
tl;dr I've used a very similar setup and it's clinically accurate.