1) If you are playing CD's on the Oppo, then driving analog cables to the Marantz preamp, you are only using the DAC in the Oppo.
2) If you are playing CD's in the Oppo, but driving digital cable to the Directstream DAC and then analog cables to the Marantz preamp, then you are only using the DAC in the Directstream.
3) If you play CD's in the Oppo and drive digital cable to the Marantz, then you are only using the DAC in the Marantz.
I would expect that #2 would sound best.
Steve N.
Empirical Audio