I generally agree with all of your opinion. Although the HD58X has a firmer punchier sub bass that grows on me for EDM music for instance. I was somewhat negative about the HD58X when I first use it. After acouple of months of usage, I find myself using the HD58X a bit more than the HD6XX, but that has to do with portable usage with my iPhone X and the lightning dongle which produces enough output voltage and power for the HD58X needs. I even use them with a new cable I bought on Amazon which includes a remote and mic. But for living room usage with my headphone DAC/amp or even for portable usage when I carry my Dragonfly Red and the Apple camera adaptor, the HD6XX is still the better, more natural, sexier/more romantic headphones. Other people may say whatever they want against the HD6XX/650 (too warm, too much mid bass, veiled, not enough treble energy, too loose bass, lacking impact and firmness and so on), but I am of the opinion that I haven’t found yet a better headphone overall for its true listening experience and in particular I truly believe that its mids are still reference standard, yet to be beaten at any pricing point.
Let me put it this way:
My HD58X (150$) + Apple lightning headphone adaptor (9$) + Newfantasia remote/mic/cable (20$) = 179$ is a great portable combo, unbeaten at this price, and fully compatible « as is » with my iPhone X or iPad Pro.
My HD6XX (200$) + Apple usb lightning camera adaptor (29$) + Dragonfly Red DAC/amp (200$) or Massdrop O2/SDAC DAC/amp (150$) are probably the greatest combos for true listening experience (379$ to 429$ depending on the DAC/amp) under 500$, but still over 2x more expensive than the previous HD58X combo. And if you take the road of higher end SS or tube amps, the sky is the limit to your enjoyment...
So both solutions are truly worth considering, depending on needs and budget...