At normal (ie lower) listening volume I also often prefer the signature of the HD58X for certain genres and (as one example) listening to TV dialogue.
As fare as "scaleabity" goes;
I don't know if you have access to a balanced set-up but if you want to truly hear how well the HD600/650 scale (in comparison to other similarly priced cans) a decent balanced amp will help demonstrate it.
If I want to listen to music at louder levels for a short period (ie a specific song) I'll reach for the HD650.
They don't break up/distort even at a volume that's higher than tolerable/safe for any length of time.
This may not be an issue for many but with very dynamic music, sometimes even moderately higher than normal listening can cause distortion when the music volume "peaks" for a short time.
I'm pretty sure this is due to the HD 600/650 higher 300 Ohms as you mentioned. But I also think there could be more to it than that. As I'm not an engineer, I'll simply enjoy it rather than try to explain why I hear their exceptional "scalability" to be actual rather than hype"
Your opinion that these cans don't scale any better than others is definitely in the minority (and I won't bother with IMO on that statement).
Nobody's arguing about or objecting to posting graphs; I was simply trying to state that two different headphones with near identical graphs along a specific audio spectrum can sound quite different.
I didn't realize with all the YMMV and "I think" or IMO that this was some kind of competition as to which is the "better" headphone. Besides, we have said pretty much the same things using different words.
They both have their strengths and weaknesses in different settings and (as stated several times already) I reach for one or the other depending on the music and type of anticipated listening session (e.g. volume,length and/or source).
Anyone truly interested can always buy both from Massdrop and hear/decide for themselve. Either pair will likely sell quickly at close to what you pay for them, although you might be very tempted to keep both
.
I'm putting this to bed but feel free to
'til the cows come home.