I've heard both (owned the HD595) and demoed the HD598 at a head-fi meet. They still aren't in the same league the HD600 is a far superior headphone and worth the few extra dollars for it.
Plus those who want extra bass on the HD600 just EQ up a few dB below 100Hz as I find it gives the headphone a lot more kick. Here's the EQ of the HD600 from what I've seen so I just fixed that dip on the left.
My sound card (Auzentech X-Meridian) has a pretty good EQ that can adjust without causing distortion when played loud. If you're not in that boat and want to see what it sounds like EQ'ing the HD600 on a Microsoft Windows machine install foobar2000 (and preferably the WASAPI plugin...although not necessary), enable the EQ (under View-->Equalizer) and on the last two bars (55Hz and 77Hz) move the 77Hz up 3 notches and the 55Hz up about 7 notches...then hit auto-level (so you're not causing distortion) and enable the EQ. This doesn't sound as good as my hardware EQ (I can EQ much lower and in shorter intervals) but I've messed with this setting on my HTPC which just does HDMI audio output and it's not too shabby. Let me know if anyone likes the extra bass kick of the HD600 this way. I know it hits pretty close to my AH-D7000 setup this way.