Now that my interest in the BP17 cubed is at a fever pitch I'm looking at it in fine detail and am confused as to how the Analog 1 input button serves also as the selector (or shift key) for the digital inputs. If you select analog 1 and don't want to shift to digital how is this accomplished? On the back panel input 1 appears to be dual purpose serving one stereo analog connection or 2 RCA digitals. I tried to figure this out looking at the B135 cubed manual but to no avail. There isn't a BP17 3 manual on the website as yet.
If the internal DAC is fitted, in either the BP-17 or the B135, the two coax inputs for Aux 2 become digital inputs, thus the Aux 2 button no longer has an analogue Aux 2 function - it becomes a toggle for digital/analogue mode. When in digital mode, selecting D1 to D4 digital inputs is done with the buttons used for other inputs in analogue mode - look at the front and rear panel images.
Look at the diagrams with explanatory legends in the B135 manual, or the BP17 manual. I expect it'll work the same way with the BP173
, supported by the labelling of the buttons and the back panel.