@audiowin55 this is a question that can be answered by either the owners of the Boulder 866 or the 866 manual.
There are two versions of the 866 - Analog and Digital. Both feature USB input. The Digital version includes the built in DAC that can be fed thru AES or Toslink. The USB input is for a memory stick or a USB remote. It will not work with streamers or computers.
The Analog version USB input is used for the USB remote.
So..,if you have a Digital version of the 866, the way to get sound from YouTube is to AirPlay to the 866 that’s connected to your network via Ethernet cable.
Which version of the 866 is it?