Edited by qwertytero - 10/22/13 at 8:00am
If you don't mind losing the mids and open airiness to the HD-598, there's always the Denon D2000. If you don't mind just losing the mids, there's the DT990. If you want to retain the mids and open airiness, there's the HE-400, but that costs a fair amount.
HE-400 would be the most ideal and natural upgrade from the HD598. D2000 will give you the deepest bass.
If you're not an extreme basshead, then you should try the Beyerdynamic DT770 or the Denon D2000. Both are excellent cans with very good bass extension and impact.
If you want something like the Beats Studio (bass overpowers everything else), well, there are bassier contenders that can be had for about half the price - XB500, XB700, PRO700MK2 & LP2.
It's fairly good budget for headphones ansd i will recommend to try D2000, they are very engaging, crisp, clear with deep punchy bass, sub bass. They are closer to HD598 in term of soundsignature but they are closed cans and offers much more bass. The other headphones to worth look at are like Ultrasone Pro2900, HD650.