Aux input, do meant SPDIF input?The Aux input on the EXOGAL Comet is an analog line input (L and R RCAs). The digital inputs (USB, SPDIF, AES, optical) can be used to connect to your choice of digital output from the Oppo.
For digital sources the Comet doesn't handle (DVD-A, for example) you can use DAC section in the Oppo, while for PCM and DSD you might likely prefer using a digital output from the Oppo to a digital input on the Comet. So you can easily switch inputs from your listening chair, along with adjusting volume. Use the app (Apple or Android), it's really good.
Either way, unless you're using other sources the ARC preamp is completely unnecessary. I suggest your VAC amp could perform even better being fed from the Comet.