Had a ground loop between my O2/Odac combo and KRK Rockit 5's. Ground loop disappears when headphones are connected. The Hifidiy USB Isolator solved the problem, unfortunately the device is not transparent, and has a negative effect on the audio quality. I will not try to describe the effect, but it's definitely not good. Out of frustration I solved the problem with Cheeter plugs on KRK Rockit 5's and returned the isolator.
1/9/15 at 6:38pm