Why would he use a ground loop isolator on USB ?

From USB since it's digital and not signal I'd say it's a different problem and there shouldn't really be noise, I'm not sure it can come from a faulty power PSU but maybe check it, and your DAC since in the end he's the one converting the signal and probably at fault ? I'm really not sure of anything but I'd say those are you leads. USB is the same protocol on all Mobo, they should all output the same data (varying according to spec like 2, 3.0, 3.1, etc.), So unless you USB port is faulty the Mobo quality won't change the output

There's really little noise to no nose on a good motherboard, also if you're using a PC prefer the jack port on the Mobo directly and not the one connected on the front