I know I didn't realize how forward the HD598 was until I had a chance to A/B them with the HD600. I'm gonna have to disagree though. Compared to the HD600, the HD598s are grainy and a touch shouty (though nothing like the K702 which I also got to A/B). The biggest improvements with the HD600 is the resolution, smooth transition, tonal balance, and lack of grain.
For example, on Rebecca Pidgeon's Spanish Harlem, the HD598 image well and place the maracas correctly in the soundstage, but with the HD600 I could pick out every little motion they made. It wasn't subtle.
the HD598s are good, but the HD600s are just better in every category, which made getting rid of them harder.