I have to second your choice on the Sennheiser cans. Although, when push came to shove, while truly appreciating the HD600, I have to say the HD650 is the phone I would not give up (and never have).
That might not seem like saying much for someone who owns but a few pair. But I’ve tried many, with a great chain of gear behind them, through meets for shorter periods and through a good friend who owns many pairs, for longer periods of time.
Although able to appreciate many sets of cans, especially the Sennheiser HD800 and LCD2. Along with less impressive pairs, for myself at least, like the rather disappointing AKG702, some higher end Grados and the Sony 3000 (plus others I won’t list here), I never heard a pair that sounded altogether better and seemed worth the extra amount of money they cost over the HD650.
One can likely tell by the list that forward sounding headphones, especially those with exaggerated top-end, are hardly my preference.
Granted, I’m not a “collector” and I’m retired and don’t have a lot of disposable income.
But I’m also somewhat of a minimalist and tend to stick with what I know and like. And I know the HD650 and like that it never displeases me.
The only thing I might miss or want on some rare occasion and with a particular album or song, would be the bass on the LCD2/3 cans, and/or the treble articulation/extension and upper mids on the HD 800 series.
But while the HD800 was tempting for it’s huge soundstage and treble extension, the lack of bass just seemed a real shortcoming in what was an expensive headphone at the time of their release.
And although the midrange might have been as good, the lower mids and bass especially didn’t seem as good as what I had become accustomed to from my balanced HD650s.
I realize one can likely get as much from an excellent SE setup, but the 650s, for whatever reason, seem to especially benefit from even a $500 balanced amp, with a decent source behind it.
I haven’t had a chance to hear some of the newer, very expensive phones (and to be honest, am not sure I want to 
After many years I still remember how knocked out I was by the Sony RS10, which at the time I heard them sold for 5000. That seemed insane in 2007 and seems only slightly less so now, even though quite a few cans are presently selling in that range.
I’ve said elsewhere that I regret selling a pristine set of HD600s (especially for $200 CAD). But I know the person who bought them was grateful to get his first pair of excellent sounding headphones at that price. Grateful enough in fact, that he insisted on giving me an extra $20 because they were in a condition that was beyond his expectation.
There is something very satisfying about selling someone something they’ve wanted (and set aside hard earned money for) at a price they’re more than happy to pay.
The HD58X I purchased fairly recently might not be quite as articulate as the HD600, but they’re pretty darn close. They might even sound better in some ways and they’re certainly easier to drive from a portable set-up.
So I don’t think they’ll be going anywhere any time soon either (and certainly not for the buck and a half they cost). 
