Thank you so much for the input. My budget is tight at the moment. If HD600 is not much of a difference from the HD598, then why would I spend another $200 for it? I think I would go with something that I can afford and I think HD598 would suit me for now. If nothing comes up between around the same price range, I'll be getthing the HD598. I really want to see how the open headphone is like.
For straight out of an ipod or computer I'd probably go with the 598s. However, if you anticipate upgrading to a dedicated amp in the future then it might be better to just get the 600s now. The 600s are a bit harder to drive, but driven decently they should sound more accurate/detailed than the 598s.
This is all my experience with them both to listen to music and play my DP. I suspect I'd probably be entirely satisfied with something like the 598s if they were just for DP.
While graphs certainly don't tell you everything about a headphone (especially not when this small and smoothed out), this does show that the 600s should have more accuracy and detail in the treble (that and they were one of Sennheiser's twin flagships for years until the 800 came out, and are good enough that they're still very popular today):
Yeah, I know it's the 595, they don't have the 598 in their database yet, but I suspect it's be similar.
for how to fix the image.