Turns out I got an OK price on 598 so am now qualified to comment on dif between 595 and 598. I also happen to have KSC75 sitting right here beside me too so know for a fact you are FOS.
Anyway, dif between 595 and 598 is a subtle difference and nothing massive so all those claiming 598 are way better than 595 are also FOS. 598 are slighty better (clearer bass) but not worth the upgrade price if one already owns 595. I shoulda held out for HD650 IMO.
I have the HD-555, HD-558 and HD-600. Oh, along with the HE-400 by HifiMAN. In regards to the HD-555 / 595, I've found that they respond a bit nicely when some small mods are performed. Instead of performing the full HD-555 to HD-595 mod by removing the foam, I replaced it with a smaller piece of square craft felt with an adhesive back. The oval ring that the driver baffle screws into was dampened a bit by using new Newplast which provides a bit of damping to tighten the bass a bit. I'd be interested in seeing how close a modified HD-555 comes to the HD-598. I would have bought the HD-598 the other night instead of the HD-558, but I also had my new HD-600s awaiting me at home on Friday, so dropping another $70.00 didn't seem like the best move for me at the moment.
There are many merits of the HD-555 / 595 that should not be overlooked. They are still a good "all-around" performer and are comfortable headphones. I paid $30.00 for my HD-555 used on Craigslist a few weeks back.