Hi,
I have a bose USB audio 5.1 device, a generic USB audio device.
I noticed that the proper channels routing was missing from the default generic USB pcm, but dmix was used. And for the surround pcms I noticed it was the other way around!
I did modify the default pcm to include a routing plugin. I put 8 channels in the routing plugin. I guess the proper way would be to detect the hw card number of channels and select a route plugin configuration with the correct count of channels (maybe their is a function to get that info?).
I linked the surround51 and surround71 pcms with the 'disable-able' dmix section from the default pcm.
I did not touch the surround40* pcms, surround pcms which are specific to some cards and the iec958 ones.
http://filebin.ca/214UTT8fBnUm/USB-Audio.conf
regards,