Hi!
I’m using the combo of Pi Zero W + USB DAC (SMSL AD18)
It definitely works. And works pretty well. (Gracias for all the people who made this possible. 
But the problem is , when the device boots with the DAC-AMP power on
Volumio screams out a huge, almost a air-raid siren-like loud noise before the start chime.
I suspect the belting occurs exactly when the device is booting.
I’ve tried other methods of booting such as booting first and then turning on the DAC-AMP and they’ve all been ok.
So… I went back and started from scratch.
This time, I plugged in the DAC-AMP before any software was installed.
(Before this attempt, I set up with the audio jack setting and later changed it to USB)
And it works perfectly.
Rebooted several times and there’s no noise.