IMO You can't begin to judge a headphone such as the 598 without a proper source and amp. Such headphones are not meant to be plugged straight into your mobo. :) The difference between an outboard DAC/AMP (I've tried a couple) doesn't even compare to my onboard audio, or even from a internal dedicated soundcard. At this point, you're not experiencing nearly what the headphone is capable of!
The E07K, as people have mentioned, might do the trick, but I've have bad luck with 2 FiiO products already (E10 DAC/AMP, D07 DAC) so I can't recommend them. Their QA seems rather shoddy.
You might consider the NuForce uDAC-3 or the Audioengine D1 I keep hearing so much about.
I don't know, I heard heaps of negative comments about soundcards, apparently they don't make any difference. Although DAC's have been widely recommended.