As discussed in this topic, I have a Steinberg UR22 mkII working. I’m using the default usb audio settings, except that I changed “hw:0” into “hw:1” (-P 70 -t 2000 -s -d alsa -d hw:1 -r 44100 -p 256 -n 2 -X raw) and that I had to make the same adjustment to /etc/dbus-1/system-local.conf as mentioned by Baggypants above.
I see no volume sliders, only a slider for “Clock Source 41 Validity” (no idea what it is for)
This is the output of amixer:
Simple mixer control ‘Clock Source 41 Validity’,0
Capabilities: pswitch pswitch-joined
Playback channels: Mono
Mono: Playback [on]