Settings-Playback Options-Mixer Type-None
When using any type of HAT on a Raspberry PI 3 this setting will disable the volume control and set the level to maximum.
With a USB DAC the volume control is disabled and reads 100, but in reality is set to 39. The reduced volume is very apparent after setting the mixer type to “none”. Confirmation of the true output level is gained when switching the mixer type to “hardware” reveals a volume setting of 39.
There is no logical reason for this behaviour especially when HAT based DACs and USB DACs yield different results.