Hi Neil, nice DAC! I had an iDAC2 for a while and really enjoyed it.
Chromecast Audio devices are now Roon endpoints. Just like Airplay devices have been for a while. Just check your Roon Core and endpoints all have the latest Roon updates.
So what I meant was you could use Chrome browser or the native YouTube app and stream (cast) audio direct to the Chromecast Audio device. And also use it for general Roon listening too, now that it’s a Roon endpoint.
Unfortunately the iDAC2 doesn’t have TOSlink input though. The micro iDSD’s and newish xDSD have a TOSLink input.
What amp are you using? Does it have a spare pair of RCA inputs? You could use the analogue output of Chromecast Audio.