I have owned my pair of Beyerdynamic DT 770 Pro 80's for about a year now, however I have been using them un-amped up until yesterday. I recently received my first amp and DAC combo (I bought a Schiit Magni and a Topping D20), and I have noticed a bit of a strange issue. At particular volume levels on the potentiometer (around 7-9 o'clock, a little behind 12 o'clock, and a little behind 5 o'clock), I hear a high pitched noise, similar to the sound of a radio being tuned, and some static. I want to say that it is somehow related to the USB, as I only hear said noise through the USB connection and not through the optical, but there is something peculiar about it. The noise is only audible with my DT 770's. I have a pair of Grado Sr60i's as well, and I cannot hear the whining and static at all through said pair. Could it be a problem with my Beyerdyamic headphones? Or should I invest in some method of USB isolation? I had read about the potential of a ground loop, but that seemed to be the case mostly for laptop users, and I am using a desktop. All advice would be appreciated. Thanks.