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.