Comment 3 for bug 1498129

As far as I can tell, ALSA is working just fine. The problem is only with Pulseaudio.

After restarting Pulseaudio with Xonar U7 attached I've got the "Xoanar U7 Digital (S/PDIF)" choice in the Audio configuration, but this choice actually selects Analog Output (no signal on digital S/PDIF output, and the sound is going through analog connectors).

The disconnect message in the quoted above dmesg is related to my experiments with attaching and detaching the USB card. It is normal behaviour.

The apparent problems with unsupported "ctl value" does not affect the functionality of the card in ALSA.
(FYI: I've added "ignore_ctl_error=1" to snd-usb-audio options in modprobe.d/alsa-base.conf to make alsamixer work)

Please, let me know what else I can do to help sort this out.