I'm not familiar with the current prices but when I bought the 598 it was listed for 180€ (I guess it should be about 160 now) and the HD 600 for 280€ (though I've also seen it for 230€ some time ago).
Okay, that's still a price gap of about 100€ but imo the difference is worth it, but even in the HD 598's price range, there are technically better headphones like the AKG K701 or Beyerdynamic DT880 Edition (or the Philips Fidelio which I haven't heard yet). The only striking strenght of the 598 is, imo, the good soundstage - it is very spacious in either width and depth (unlike the K701 wich is only capable of width or the Beyerdynamic which has got a rather small soundstage though it is 3-dimensional).
I wouldn't call the 598 bad but not worth the money you pay as there are better 'phones for ~200€. Despite, you get to hear the typical Sennheiser sound with them.
Imo, the HD 600 is the Sennheiser with the best money-to-sound-value. Sure, my HD 800 obviously beats my HD 600, but the sound is not four times better and the HD 600 is the more reasonable headphone.
I own K702 and DT880/600 and couldn't disagree more with your assessment. For example, the Beyers are capable of throwing such an enveloping soundfield (not soundstage, headphones can't produce one) that sometimes I have to take them off to make sure the speakers aren't playing.
Each one's perception is different, especially the one of how one perceives the soundstage (yes, I'll call it that way as I can get some kind of speaker-like experience, depending on the headphone, if I close my eyes, but it is closer to my head than with speakers for sure). Without hrtf-adjusted dsp and equalization, one will not be able to hear a real out of the head experience with the soundstage being built up before one's head with headphones though I perceive some neutral iems to come close to it.
At least to me, the DT880's (also 600 Ohms) soundstage is three-dimensional yet small (in terms of width) in comparison to the HD 598/800 or K701.
But as I said, I feel that the biggest disagreements between different people describing the same headphone come up when they are about to describe the soundstage (/enveloping soundfield).
Nonetheless, Imo the K701/2, DT880 and hD600/650 are good in their own way and the HD 598 is one step below.