Mr Trev
Headphoneus Supremus
If you are using it with this firmware (3.34b), you can't use the DAC with Ubuntu 14.04. It could maybe be they case that it doesn't work with Linux at all. It is only 3.34b that the DAC has stopped working with my PC. If you have windows or an older firmware, the DAC should work just fine.
I had mine working with the latest version of Mint, so it should work in Linux. Actually it was pretty nice, no drivers to install, no latency issues like I have in windows. Just too bad I couldn't get my most used Win programs to work properly (Musicbee, esp.) and went back to win7.