Thought you should see this list:

http://www.silentway.com/tips/equip/io.php

Just a few options... wink

Of course, all of them are going in the wrong direction for what you're looking for. The only way I can think of that would work for you would be to take the output of your system and use it as an input on another computer. Then the second computer would have the proper drivers on it to drive the Soundsticks through USB while the first computer outputs the SPDIF. I guess that's a little too much to drive some USB speakers. For the price, I would pick up a pair of the AudioEngine A2 speakers. I heard them the other day and they sound way too good for their size.