My feeling if you have the asus xonar essence stx is to stick with it. The O2 is amp and if the output is too high it will overload the speaker amp inputs before it even gets to the outputs. There can be a impedance mismatch since it is not a pre-amp per say Using an amp to drive another amp is not always the best idea. I can give more tips later when I done the Emotiva mini-X A-100 installation and listening tests.
I see. Well thanks for the advice. I thought that I could use the o2 as a preamp because I believe the OP used an amp as a preamp but maybe I misread.