I have my streamer connected to a DAC. You use the optical/spdif out from the ML Unison to the input of your DAC. Once everything is hooked up THEN run ARC and you are all set. See page 8-9 of the manual. Anything you connect to the Unison inputs (RCA or optical in) will be passed through to your DAC and get the benefit of the ARC room correction. For example you could connect a turntable to the RCA in and a CDP to the optical in and they all will pass through to your DAC if you want via optical out OR you can use the RCA out to your preamp. No matter how you connect it everything will benefit from ARC, Just remember, you have to run ARC after connecting your DAC. That review/article is from 2018, both ARC and Play-Fi have had many software updates since then:
https://www.martinlogan.com/uploads/documents/manuals/manual-unison.pdf