I just got a Denon DA-300USB DAC, which requires drivers on the PC side but claims to work on the Mac without them (not unusual). This of course also means the user guide is useless, and seems to think things will Just Work(tm), but when the DAC is configured for USB input (using the little picker on the front-panel) it says "Unlock" when there is no audio, and "Unsupported" any time I try to play audio.
I don't think this is just a DRM issue with iTunes audio, but I can't find anything on google to suggest what I should tweak (it shows up fine in the sound control panel, like my previous BitHead DAC).
Best Answer
It turns out the answer is reasonably simple, and completely obtuse.
You need to open the "Audio MIDI Setup" application (in /Applications/Utilities, if you haven't moved it), show the "Audio Devices" window (if it isn't open already), and choose the USB High Speed Audio device (hopefully there's only one - it should show '0 in / 2 out' for the DA-300).
The default format for output is 352800 Hz, but the DA-300 only supports 192000Hz. Simply change the format popup for Hz to 192000 and you should be in business. Both 2ch-24bit and 2ch-32bit sound work, although depending on your source data you may be happier with 24bit.