Well at the moment I do run an Asus Xonar STX as I only listen to music from my PC. Now, I think it sounds great, and I did pick up a Matrix M-Stage at a steal used and saved a lot of money. Won't lie, took a little bit to fine choose out the differences, but it's definitely warmer, pulls out more details without having to go obscenely loud like my Xonars built in AMP. (I already knew the STX's weak point was it's AMP.) Though I've always read and been told a good OPAMP swapped STX still stands strong as a DAC. Now my question is how big of a difference would it be over a modded STX to say a 200-400 dollar stand alone DAC?
Also for reference, I currently have HD595, HD650 and soon coming HE-400s. In love with the Sennheisers taste(sound) so far. Hifimans I hear tackle the sound in a different way and curiosity made me bite the bullet on trying some HE-400.