I Also got the Tiamat 7.1 and have the same issues as many people describe.
A constant background noise in the headphones when i plug them in my usb port.
I also tested an external AC USB Adapter, USB Hub, Carradio-USB-Slot, Receiver-USB-Slot, PS3-USBSlot. Nothing worked.
Is it normal that there is a difference if you compare the sound when the headphones are plugged in or out. Or should you hear exactly nothing, if plugged in?