I think there is a bug with the DAC2HD driver, which results in the device volume being set to 0 (and resetting itself if you try and move it).
My work-round involves using two devices adjusting the endpoint while it’s playing.
I sent the volume control to device volume. I then hold it on maximum with one device, while changing the setup to dsp volume with the other.
With luck (and a few attempts) I end up switching to dsp volume while the device volume is maximum.
I then never change back to device volume. This seems to then be OK as long as I don’t reflash ropieee.
No idea if this will work for others…