Correction: When plugging the headphones (through the amp) into an iPod, there is no ringing, but when I plug it (again through the amp) into the usb hub there is a very quiet whistling/high pitched hum. I am using the amp now without plugging it in, and there is no background noise.  I...

I just did, and it seems that it only rings when I put the e6 next to my usb hub. Is this some form of interference?

I've had my DT880 pros (with a Fiio e6) for several months now, and I've recently noticed a very quiet but very noticeable ringing noise. At first I thought it was just my ears, but when I lifted up one cup off my ear, I could hear the ringing in the other ear but not that one. It seems to go...