The MG10 is not USB. The MG10XU is USB. If you use the MG10XU you really would not need the Xonar Essence STX for this particular purpose. You can connect your mic and your speakers/headphone directly to the MG10XU. The MG10XU "should" show up in your soundcard properties as an audio interface. You will probably want to keep the soundcard properties in a quicklaunch in your task tray so you can make whatever adjustment there required to prevent the output from recording back to the input and making a horrible screech or compounding volume escalation. This is normal to switch it back and forth in the soundcard properties.

Option; Use the MG10 (not usb) only for it's phantom power and preamp; Connect your speakers and headphones to the Xonar Essence STX for playback and monitoring. Plug in your NT1 to the MG10 to be used for phantom power, compressor, EQ, preamp only. Plug in the MG10 output to the input on the Xonar Essence STX. You can choose any output you have an adaptor for. Even the MG10 headphone output. That would keep it very simple and you should not have any internal loop issue this way.

There are a few other options but I'm out of time.

Does the noise in your head bother me ?