This has become a topic of great interest to me. I record the USB output of my Digitone onto my computer. I am shooting for a nominal sound-pressure-level for the resulting audio files. I find the results ca vary from one project to another. I suppose this is because there is a distinction between SPL and humans’ perception of loudness. I know there are other tools available to change the SPL, to normalize, amplify, clip, whatever, my tracks…but I am trying to avoid them. Partly because, it seems, the Digitone has all the tools I need (level monitoring would be nice; can we please see that in an update?).
Though I don’t have experience with other boxes, I think I understand what some people mean when they say the DN has massive headroom. The music I make is on the tame-side for the DN (accompaniment tracks for students to play along), and I wonder if that is why I frequently end up with an SPL a little on the low side.
Edit: You mentioned Main Volume as an element of gain staging. All I have ever done, output-wise, is record out from USB, and in this mode: Main volume has no effect on output USB sound level.