I just got a Sennheiser HD 598, and was planning on getting a Sound Blaster Z. Will the onboard amp on the soundcard work well with the 598's? What are the benefits of getting a standalone amp compared to the one on the card itself?
The Sound Blaster Z is plenty strong to drive a pair of HD 598's. That being said, you may find the sound more acceptable to your own preference by using an external headphone amp. Not that it would be objectively better per-se, just a different coloration on the sound that some people prefer.