I have a late 2014 Mac Mini with USB 3.0 running OSX El Capitan 10.11.5.
I have never used the usb interface function before but just tried it out and it works fine.
The amp has all the recent firmware.
The Amplifi 75 does not show up as a drive icon on the desktop but it does show up in the Audio MIDI Setup app.
I opened Ableton Live 9 and the Amplifi shows up in the Audio Preferences pane selection menu.
It configured from there and routed audio with no problem.
I even used a usb hub and was not going directly into the back of the computer.
It is frustrating to not have things go smoothly. I don't know what to suggest except check usb cables and ports.
Perhaps I don't really understand your situation, maybe it is an issue with Macs after 2014, but it might be worth reverting or factory resetting and then updating again.
Someone more knowledgeable can hopefully suggest some solutions.
