I have a basic rpi4 Volumio player setup, which is connected to my Hegel H90 via a USB connection. This has worked from day one, but after an update to the newest Volumio I can’t find the right setting for the output. I have tried all the generic ones and none of them let the sound through to the amp.
Can anyone help, thanks 
Reboot router. I bet it shows up again.
Settings/Playback Options/Output device needs to be set to USB audio CODEC. After going through the same fire drill a few times, I finally figured out that’s where my problem was. Maybe this will help.