There is a coax in on that GaN that would come from your DAC. In the case of the Bluesound, it has an internal DAC. However you could insert an external DAC between the Bluesound and the GaN. Volume control could be via either the DAC or the Bluesound depending on the DAC. (I have a Border Patrol DAC so manage volume via the Node2i when streaming)
If you're pure digital I can see the utility. OTOH if you have an analog source like a TT, you're SOL I think.
Happy listening