Tiamat v2 USB Power Without Computer / No PC

Hi, I'm trying to work out how to get these to work without the PC on.

As far as I can see from the site the USB just provides power to the headset, which is all I need, as I'd like to use the headset to listen to films as it's plugged into AVR pre-outs, and I'm getting a sumptuous gaming performance from that.

However, I can't seem to get sound (indeed the 7.1 light will not come on) unless the USB is plugged into a PC that's turned on. This is less than ideal, but am I just not using powerful enough USB slots?

I've used the (powered) USB slot on the front of the AVR, and I hear the speakers "chck" so I know power is flowing into the unit. However I get nothing from the sound.

I also tried a DC 5V 1000mA output socket on a plug, to no avail.

Does anyone have solutions here? Raspberry Pi?

Any solutions will be gratefully received!