Need help with new set-up.

Here is one way to do it:
Buy a converter to convert from what you have on your output on your soundcard to XLR.
You might not get balanced between soundcard and subwoofer if you don’t have a soundcard with balanced out
This should not be a problem if you don’t need very long cables to connect the subwoofer, try to get the subwoofer near your soundcard and as short cables as possible. Then XLR from Sub to top speakers.
BTW My soundcard (M-Audio Fasttrack Pro) does have balanced XLR out…..

//DJ Hessler