your sound card has 1 digital IN and 1 digital OUT. Why don't you just set your mixer's digital OUT to the the digital INput of your sound card.
Also, your mixer has two more outputs (rca & xlr) in addition to the booth output. Your sound card also has more outputs. Use whatever you want and don't power any speakers you don't need when not using.
Hope you get what I mean.
I would use digital for recording by the way.