If you want to use a compressor when tracking, hook it in to the insert connector of the ART DPS-II pre-amp. This takes the signal out after the pre-amp and returns it before the A-D converter. Don't use much compression at this stage - it's not easy to undo when mixing.
You need not be worried that the insert jacks on the DPS-II are unbalanced, as the compressor has only single-ended output, dressed up as pseudo-balanced. Watch out for the pre-amp's insert jacks being wired the unconventional way, so if you use a standard insert cable, interchange the usage of the "send" and "return" TS plugs.