Plug the USB to the PC from the DAC. The DAC plugs into the headphone amp. Your headphones plug into the amp. Windows recognizes the DAC as an audio device. I use the Razer surround software. Simple and sounds great. In Windows you choose the Razer Surround Audio Controller as your default device. In the Razer software you chose that DAC (in the screenshot below the MODI Device) as your audio interface. You now have the clean sound of an external DAC/AMP/Headphones...with the features of the Razer software. EQ, Surround etc.
You mentioned you want to see what all the hype about the HD6xx headphones are about. You're basically getting some 500 dollar headphones for 200 bucks. They are worth 500 bucks IMO, so at 200 that is a crazy value. The sound from onboard is OK..a 4 or 5 in sound quality? These headphones and amp/dac/software bring you to a comparative 9. 10 would be some HD800's or something.
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