Hey Jamie – we just discussed this and our understanding is that, unfortunately, the Audio Research Reference CD9 is not compatible with Linux, so it’s not going to work with Nucleus, or with Raspberry Pi running Roon Bridge, etc.
You should be able to get this working with a device that outputs S/PDIF, which can plug right into the CD9. We’re big fans of the Digi One from Allo, which has been getting some great reviews and would offer some additional isolation between your Core and your DAC. There are a bunch of good S/PDIF options out there (both USB and network-based), so I’m sure you can get some recommendations here on Community…
You might also check with Audio Research – I don’t know if they have any plans to add Linux support for this DAC, but it’s worth letting them know you’re interested.