So I've been struggling with this issue for a little while and I can't seem to figure it out. I am on a clean install of Ubuntu (meaning I haven't messed about with audio stuff yet), and I am facing the same issue with my Dragonfly USB DAC as I have with other distros. When the volume is below 100%, the sound coming into my headphones through the DAC is barely audible, even when close to 100%. However, if I push the volume over 100% it becomes very audible, too loud in fact.
Is there a known "fix" for this issue? I'm assuming there's some way to get around this since the audio does play through the DAC, the issue is just the large jump from soft to very loud when going over 100% volume. When not using the USB DAC, the audio is as expected. I can comfortably listen at around 30% volume.