I don't think sound cards have low level outputs? At least not a SB.
I'm doing a stereo system in my office too and I'm actually using my digital outs to go to a receiver and then a reciever to the stereo. Will full range out going into low level in hurt things?
I think what you have to do is use the analog output of your sound card. Looks like you would need a 1/8th jack y splitter into 2 rca female ends. And then from there run 2 rcas into your your line level input (red white rca plug) of your sub. And then run your speaker wire from your sub to your speakers.
However, I don't think you can do surround sound with this type of setup, because of the in put out put set up on the sub amp.
Which gets me to thinking about what setup I'm going to run on mine. Does anyone know if it's harmful to run from asound card to a amp or receiver since in fact the sound card is a small amp its self, and has power coming off of it?
I'm not an expert and prolly know the least on here, but I hope this helps get you going.