Connecting my Galaxy S6 to a Fiio Q1 (via OTG), I've noticed that the volume control on the phone still works (I'm not using any app at the moment, just native OS).
Firstly, shouldn't the volume control on the phone be bypassed in this case?
Second, I understand that many adjust the source volume to around 75% (or just below max) when using an external app. Does this same rule apply to the "digital" volume control that's going on when connecting to an external DAC? In other words, should I set the phone volume to 100% in this case?
Yes, set the digital volume to 100%.