I would consider the Magni headphone amplifier a little better then the amp built into the ZxR or STX.
The Xonar DX and Essence STX use the same C-Media CMI8788 audio processor.
So buying the Magni (or Vali? or other amp) and plugging it into the Xonar DX should work well for you.
This may sound silly, but is it possible using an external DAC and a soundcard at the same time?
I could hold onto my Xonar DX and make it go through a Magni for when I needed surround, and when I am only needing stereo, I could just unplug my amp from the soundcard and plug it into the external DAC.
Also, what are your guy's thoughts on Magni vs Vali for the AKG K712? I saw that Magni has below 1 impedance and the Vali is like 6 impedance. Which one would be better for the 63 ohm (or power hungry) impedance of the K712?