Thanks, Mario!
@David_Snyder advised me to exclude SoundSource. And of course he was right. Roon works flawlessly without it.
I really do like to play other sources - especially Dorico (for realism’s sake) through the room’s stereo.
I could live without YouTube etc; and listen on my iMac’s speakers etc.
Would a USB switch like one of these, allowing me to select whether the output from my iMac or the output from the Nucleus went to the P6 (or Streamer) cable degrade or otherwise compromise the sound, do you think?