Can't play audio to Denon DA-300USB DAC
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.