I don't agree with this and that's a bit overkill, but just my opinion. I just had to comment. I do love the HD-598 though! I think overall the HD-650 is the better headphone, but the HD-598 is generally more fun to listen and a better match for my music. Of course this all depends on the amp and the cable. I really disliked the HD-650 out of the Asgard and on the E9 it's rather dull and lifeless, but not as much as the HD-600 (with stock cable). I think you really would have to compare the 598 and HD-650 side by side.
If you mean an upgrade in some areas, then I can understand. To me the HD-598 has better imaging, comfort, a larger soundstage, a more neutral sound and more engaging mids. The HD-598 is easier to drive and like though. The difference between sound clarity and the level of detail between the HD-650 and 598 is like night and day to me.
I think it all comes down to preference. The HD-650 for me seemed to be very very picky about amps. I didn't love mine until I got my Micro Amp. Same with my K702 until I bought the E9. I do think the 598 is better for more people and is much easier to like. The HD-650 is still not perfect. I wish it just had a clearer sound that was as clear as the K702. Still not too bad. BTW I had the silver screened version and I tried the old version years ago and hated it.
I attempted to go with the HD-600 instead over the 650, but that thing put me to sleep, but I did love how more neutral it sounded compared to the HD-650. I'm not a huge fan of the bass on the HD-650. HD-600 was also far better for movies and games than the 650.
I can't wait to see a K601 vs HD-598 review. I could do one but I hate over-analyzing my headphones these days. It stops me from enjoying them and it gets old fast.
BTW I would say the HD-598 is an upgrade to the OLD (non-silver screened) HD-600. Someone will hate me for saying this, but it's true. I do think the HD-600 (silver screen) is slightly more clear and less grainy, but it sucks the life out of music compared to the HD-598. The HD-598 also feels brighter than the HD-600 until you upgrade it to the HD-650 cable.
Well, let's clear that up just a bit. I've heard of the HD650's insane potential with the right components, mainly a stellar amp. With that, no doubt they would smash the HD598. But from the basics, or cheaper budget amps (MKIII for instance), I find the HD598 an all around better headphone. I will admit, I like the HD598 in a majority of aspects over the HD650, but not all. No doubt the HD650 wins on clarity and dynamics. But then again, the margin of difference isn't that huge either. I'm finding the term, "detail" to be one of the hardest things to judge, because I'm still hearing everything with my AD700 or FX67's compared to my DT880, HD650, Pro 900, or RS1i. I think with detail, just comes the term, "presenting the details". Clarity is probably a better word in my opinion. In that case, the HD650 does win out on that, and it does win in the bass as well. But the HD598 wins on price, soundstage, comfort, sound signature, mids, and energy. The HD650 with the right amp and cables? I couldn't imagine. But I wish they put that sort of technology in the form of the HD598.