What should I configure to get this DAC working?
What other relevant info should I provide? The DAC is known to be working (works fine in OSX and Windows).
I recently got mine working but I could only do so easily using pulseaudio. Use something like pavucontrol (sudo apt-get install pavucontrol) to control which soundcard to use by default.
In pavu, under the output devices tab, click on the green checkmark button lined up with Schiit USB Audio Device Analog Stereo. Now play some music and enjoy.
Like the OP, I also could not get the Schiit Modi DAC working with Ubuntu (12.04). Specifically, the device was not listed in pavucontrol. Here's what worked for me:
Installed the 3.11 kernel
sudo apt-get install linux-image-3.11.0-24-generic
After booting with the new kernel, my nvidia driver (and therefore video) was messed up. I needed to install the 331 nvidia driver for compatibility with 3.11.
sudo apt-get install nvidia-331
Hope that helps.