The issue could very well be El Capitan, notorious for usb audio problems and still unfixed by Apple in Sierra. Google "forum el capitan audio problems" and you'll find a ton of unhappy customers. A couple of weeks ago I bought a new laptop running El Capitan and it doesn't allow me to control the volume on my USB DAC, even though it worked perfectly in Mavericks. I downloaded the app "Sound Control" that provides a partial fix but still can't run Audirvana in Integer Mode, so it's basically useless for me. The only hifi way to fix it is to go back to an earlier version of OSX which is not for the faint-hearted. My current workaround is to use my old laptop running Mavericks which I was planning to sell.
Incidentally most people who have tried all the interface options think USB and SPDIF provide better sound than optical, despite optical being better in theory. Also if you're new to computer audio stay away from iTunes, it sounds terrible for various reasons. You can download a demo of Audirvana for free and compare the two; it's night and day, not even close. (You have to turn on Integer Mode 1 in Preferences).
Incidentally most people who have tried all the interface options think USB and SPDIF provide better sound than optical, despite optical being better in theory. Also if you're new to computer audio stay away from iTunes, it sounds terrible for various reasons. You can download a demo of Audirvana for free and compare the two; it's night and day, not even close. (You have to turn on Integer Mode 1 in Preferences).