Two ways you can do this:
Xonar STX > RCA > AMP > Speakers & Sub
Xonar STX > SPDIF > DAC > AMP > Speakers & Sub
A high quality DAC would give you the better quality audio, but depending on your choice of music and speakers, you may not notice the difference in clarity.
Xonar STX > RCA > AMP > Speakers & Sub
Xonar STX > SPDIF > DAC > AMP > Speakers & Sub
A high quality DAC would give you the better quality audio, but depending on your choice of music and speakers, you may not notice the difference in clarity.