Hello,
I'm new to the forum. I have been using MC since it first was available for Mac. Back when MC 20 was brought out, I installed it, I had the same problem, distorted low volume sound. So I reverted back to MC 19 which work fine. That's why I never upgraded to MC20 or 21.
Upgraded to El Capitan and have the distorted low volume sound again. OS does see the usb DAC, have sound when playing through iTunes, have system sound through usb DAC.
Dac is a Cambridge Audio DacMagic Plus from a Mac Mini 3.1 Core 2 Duo 2.26Ghz 8Gb.
Tried reinstalling, Midi settings, MC Options, System settings to no avail. Tried different usb ports. Thinking of using the Optical Digital Out on the Mac Mini.
Hopefully it will be sorted by one or the other.
Regards,
Dingwall.