If you use a pc. Just buy a relatively cheap, but still very very excellent O2/ODAC. From either Mayflower, or JDS jabs.
If you want to connect to your av-reciever, you could consider spending a bit more.
This version has RCA in AND out. This means you can plug any device into these yellow and red ports, to either input audio or output.
This is on top of usb functionality. so you can hook it up to your phone/pc and av reciever. (assuming your av reciever has rca out, which is very likely).
I use this one, and i am very satisfied. (also use HD650's)
What this little box does, is amplify your headphones(more then capable of driving the 650's), AND it neutralizes the sound. Meaning that music/videos etc, will sound as the moviedirector, or musicartist intended you to hear it.