Thanks for the responses, folks. The answer in my case was to switch the USB setting on the Esoteric from HS (High-speed) 2 to HS-1, which bypasses the clock single on the DAC and reverts from asynchronous to synchronous mode. I haven't listened super-carefully yet (or to anything above 44.1), but on casual listening to low-rez files, I'm not hearing any difference.
Another issue (gotta love Apple!) is that with the Yosemite upgrade, the Esoteric high-rez player (downloadable at http://www.esoteric-usa.com/HR-Audioplayer.php) can't find the K-03 as the output device, despite all the settings being correct. Sigh . . .
Another issue (gotta love Apple!) is that with the Yosemite upgrade, the Esoteric high-rez player (downloadable at http://www.esoteric-usa.com/HR-Audioplayer.php) can't find the K-03 as the output device, despite all the settings being correct. Sigh . . .