can I output redbook CD's via HDMI from the 103D and use the I2S of the DS or Pontus2?
Unfortunately no. The HDMI Audio out of the Oppo is only for Dolby/DTS formats and does not support i2S. To get i2S you’d need to get a DDC with an SPDIF input like a Denafrips Hermes, but that costs almost $1300 so not an insignificant expense. But regardless, something like the Pontus 2 or Direct Stream DAC should be a significant step up in performance even using an SPDIF connection from the Oppo or Marantz. Hope this helps.