If you go with an interface be sure it has phantom power for your condenser mics.

Your Xonar Essence STX, although a very nice soundcard, is probably not ideal for audio recording/mixing. Not that it won't work but you will be switching out adaptors and cables all the time and I think you'll need phantom power.
If the STX is connected to your home stereo via Toslink and you have a remote control for the stereo I would certainly leave it in there and use for playback, listening, monitoring, etc... Or, if you wanted to just keep it as your audio interface then adding/connecting a small mixer with separate group and main busses would be fine in my opinion. But, like I said, you'll need to be able to record one or two channels to your computer while the playback from your computer is not getting recorded.

My MG124CX mixer is used all the time/permanent for connecting all sorts of playback devices, instruments, switching back and forth between monitor set ups. I have 3 different monitor set ups and sometimes a forth which is JBL powered speakers so I can get an idea what I might sound like in my live performance set up. All that of course is not necessary just that I have it all and excess cables so why not. I have the output of my interface going into the MG. The MG does not connect to the input of my interface. Anything I record gets plugged in to the interface for a clean signal path.

I'm pretty sure the MG10XU will act as an USB interface and mixer for you and give you the routing options needed.

Last edited by Tobias; 02/11/17 10:00 AM.
Does the noise in your head bother me ?