The X12's use USB to power the integrated headphone amp, and there are 2 x 3.5mm jacks for audio and mic.
Friend has same problem on PC and was not able to resolve it. I have same problem on Xbox 360 when I had the X12's and DSS2 decoder. Tried powering from pc, powered usb hub, laptop (battery and mains), 500ma and 1000ma usb chargers plugs, tv's usb ports, monitors usb ports Hiss would not go away. Brought it to friend house. Same story. So not a ground loop.
Turtle Beach support told me it's the integrated headphone amp and is normal. Though my friend's hissing was no where near as bad as mine. In the end I just stopped using them. I also had the X42 wireless headphones and they hissed too when I plugged in the usb connection. All Turtle Beach headphones which are usb powered seem to have hiss, though you can get lucky and get one which has very little hiss. Forums are full of people complaining about this.