Under 13, XBMC had a volume control that let you adjust gain during video playback (not music playback, IIRC.)
So it's possible to adjust the gain of the digital output - but it's important to understand that it is a digital output, whether via HDMI or optical out, and many apps simply will not touch a digital volume - doing so in some ways defeats the point of digitally reproducing sound since in theory you want to avoid altering the signal in the digital domain, you want to get it to the analog gain stage untouched, and then increase the signal amplitude there.
Ah, figured it out:
You need to disable passthrough in the audio menu of the video you're watching -- failing to do that means that you're telling the system 'pass through an exact copy of the audio' - which precludes fiddling with the gain of the signal (and thus compressing it.)
Once that's done, the gain slider operates.