It depends. Only if your current soundcard is really crappy and you notice interference or the sound is just bad. You can pick up the Modi for like $99, so about $200 for the modi/magni if you're on a budget and wanna keep the 650's. If you don't wanna invest an extra $200 then just return them or sell them.
Yeah, my soundcard is pretty crappy and noisy. Thus why I run everything through my FiiO E07K, but those are not enough to drive the HD650.
Thanks for the info Zalithian, I will have to think about what to do....
No problem. Good luck!