The pre-amp goes between the sound card and the DCX. Most sound cards seem to have a very low output level, which can be a big problem connecting to a pro audio dvice like the DCX that expects reasonably high voltage input signals. However, if you get a pro audio sound card with balanced outputs and higher voltage output, you could skip the pre-amp. As for specific recommendations of soundcards; that's not my department - I don't keep up with those. But one example is the E-MU 0404 USB, which has all of the features/options needed to avoid needing a pre-amp.
-Chris