I asked this recently.
Get a Xonar soundcard. Anything from the DGX to STX, depending on how much you want to spend. Then hook that up to a Schiit Magni amp. You can get both from Amazon/Newegg.
EDIT - Actually, if you are getting a Schiit Magni, make sure you are either getting a soundcard with an RCA output or know that you may also need to get a 3.5MM to RCA adapter if the soundcard does not have RCA out.
Does the STX even really benefit from an amp though? It's plenty strong as it is.