Need Help on Sennheiser Headphones

Okay so I think I'm ready to buy my first audiophile headphone. I have decided on Sennheisers after listening to the very promising HD428 (their noise cancelling, "super-bass" closed headphone) which I converted to a removable cable deal that sounded much better. Before I bought my headphones, I thought I'd ask for advice from you guys. So my first question is, how is the detail on the Sennheiser audiophile cans? (HD598 and up) On the 428's, the detail is very good after the upgrade, but I still feel like there is a whole world of detail that I haven't been able to hear yet. Next, how is the bass? It would be helpful to compare the bass to some brands like Beats or Sony closed headphones because I've never gotten to try open headphones. (yes I know they don't have Beats bass but I'm just using that as a comparison point) On the 428's the bass was taken back a notch and was much more detailed, but on rap songs, it just sounded very hollow. I wouldn't want this to be an issue if I buy the open headphones. Lastly, how is the overall listening experience on open headphones? The 428's seemed to have a very wide soundstage for closed headphones. I don't know if I can take much larger of a soundstage because I'm afraid that the sound won't be up front enought to hear everything. So any input on these questions would be great. I would also like to know if there are any significant jumps in sound quality between the HD598 and HD600, and between the HD600 and HD650. Thanks!   (also I will be buying a fiio amp for whichever headphone I get)