I find similar impressions regarding 599 vs 58X and 650. HD599 is not the most resolving, but I prefer them the most since they've got the biggest soundstage - it's like the mix of best worlds width from 598 and depth from 650, and punch the most at the same time integrating microdetail retrival almost at the same level as HD6-series. The latter can be better with some amps (again more resolving) and specific types of music, but to me 599 is the most versitile and well-rounded. They fixed everything people were complaining with 598, which were distortions at high volume and wide but tunnell-like soundstage. Again 599 is at the same time wide, but this time incorporating that "3D blob" effect from HD6-series. This particular feature along with improved bass kick made for me HD6-series absolete. It's comical how similar they are and how big of a price difference there is. HD599 are frequently dropped to 99$, while 600, 650, and now 660 are triple and even more the price. Not to mention HD500 rule them all in comfort department and quality of materials used, especially compared to massdrop releases. HD58X is a joke having driver derived from HD5-series, but with HD6 tunung. And for the worst, because they don't resolve as others HD6 do.
Having said that... I have to warn people to be careful about sennheisers as quality control is very inconsistent so the unit variation is high. I've had 5 units of 599 and came to the conclusion there's basically two sound signatures - one being more aggresive, better bass impact, darker tonality, and the other less bass, flatter boring tonality. Since i prefer more dramatic approach I stick to that one. If you buy one and don't like it, please order replecement. Maybe you will get a pair with tuning I fell in love with and stopped thinikg of going back to any of the HD6-series.