I believe I have an issue with a ground loop. I'm looking at some USB Isolators. I found this,
Would this eliminate ground loop? I have both my both two outputs going to my mixer from my USB DAC and also from headphone out?
That device is built around the ADuM4160. I have used that device to isolate USB and it works great, however I have never used it for audio applications. It should break a ground loop, assuming that is the problem that you are hearing.
Note that the ADuM4160 max speed is 12 Mbps. Some USB devices expect to be able to function at 480 MBps, but an audio interface should be fine with the slower speed.
I prefer not creating ground loop, rather than filtering it on a single connection. The nature of ground loop is very simple: your setup has to have only one ground wire (remember that every data cable has that wire) to each device attached and only one exit to the ground. Imagine a star built from ground wires where each beam has to be represented by a single ground wire. In short, from all devices attached together ONLY ONE has to be grounded to the wall socket. All the other should not be grounded to the wall sockets, because they already grounded by the data cable to that "central" device.
I hope, this will help you and save some money)