Ok, tried again with AES/EBU output and still no front knob volume control. I have latest firmware, tried usb and AES/USB input. Only using standard XLR cables, not 110ohm, but no digital dropouts etc.
I did control digital volume with GLM - to be honest not convinced I can hear any difference over analogue connection to my Genelecs.
My unit was an early one #31. Wonder if there is a hardware difference?