HD599 might offer an improvement on clarity and bass extension.
In my opinion, if you actually enjoyed the sound of these cans, you might find it hard to migrate to something else.
Going the HD600 variant route might seem as a logical step, but it is not that easy...
I made that move and ended up keeping my HD600 for collection reasons, in their box.
Can't compete to the HD599 for my ears, music preferences and equipment.
Soundstage and overall depth is missing and they are tuned differently EQ wise.
Then I tried the another 'logical step' getting the HD560S.
The enclosure shape is the same as the HD599, so I was pretty sure they wouldn't suffer from a collapsed soundstage like the HD600.
I was right about soundstage, but ...sound signature was way off.
It was bossted in the mid to higher octaves. Cold and lifeless. Some people like it, some not.
Whatever I did (cable mods, adding foam insulation and patches to the enclosure and applying heavy EQ) could not ameliorate its shortcomings.
The drivers used are much different. One final mod I have planned is to change those damned drivers and using the drivers of the HD599.
Just to see what a proper driver would sound in their enclosure.
So, my advice would be to get a pair of the HD599 (they are really good value at the moment) and look to spend your money in a headphone amplifier.
You can also upgrade the performance of the HD599 by cable modiing them.
Have fun and if you try any other cans, report back with your experience.
I would like to hear it from someone who can compare it to the HD598.