It is possible to use any available output of a FireWire Core Audio interface from within macOS. For example, if you wanted to play iTunes out from the Orpheus S/PDIF outputs, you would do this:
Connect Orpheus and go into Audio MIDI Setup (AMS) in macOS Select 'Properties For: Orpheus' Go into 'Configure Speakers'. You should have a list of all the available outputs of Orpheus listed. Check the box next to SPDIF (outputs 9-10) Below this list is the speaker configuration, make sure you have stereo selected beneath each speaker you will be able to select the output depending on which outputs you checked in the above list. Select S9 for the left speaker and S10 for the right speaker.
After making changes of this kind, you may find that you need to re-start your audio applications in order for the changes to take effect.