You can try to SSH into Runeaudio, and then try
alsamixer
And then push F6 and choose the USB device. and then try to push M (mute/unmute)
I have a problem with one of my dac´s, and each time I connect it, it automatically mutes. you can see if it is muted in the mixer, the values are then "mm"
I know that it is possible to save the settings by typing amixer and a letter, but can't remember which one. but im sure that Google has the answer
Regards
Rune