I still think the HD6xx are great headphones at the knee of the price/performance curve. But for me personally, they are not "as good as you need to go".
The above is really well formulated.
What I found over the years is that EQ is really necessary for headphones. They can easily vary 10dB here and there. Much more so than speakers.
One would think that if one EQ's them so they sound the same so one could get away with a cheap headphone, but this really is not the case.
There are still substantial differences in the 'dynamics', 'smoothness', 'reality' of how instruments are 'portrayed', distortion, roughness of the treble, resonances, lack of 'airyness', cymbals sounding differently.
Things like driver size, materials used, angles, pads, seal, porting, driver ears distance, resonances (in cups and/or drive) all make audible differences that may not stick out that much in relatively 'basic' measurements.
I generally find that SQ improves with price and do think that the LCD-4 for instance is 'better' than the LCD-2 but the question is if the difference in SQ is worth the price increase.
But not always the SQ increases in price. Utopia (4k) is not 'better' sounding to me than the Clear (1.5k) and the HD700 is not better sounding than the HD650 but the spatial clues are 'better' and so is the comfort which might be the reason some prefer the HD700 over the HD650 but others sure see it the other way around.
Then there is longevity, how long one can get original service parts, how sturdy certain parts are, how scratch/drop resistant some headphones are.
I have some cheaper headphones (all modified or EQ'ed) that I really like and have 2 or 3 prototypes I got to keep that sound great but never saw the light of day or they skimped somewhere and ruined the sound.
For me personally there is one favorite that hasn't been beaten yet (but some came really close) but needs EQ.
But as said.. most do but folks dismiss them easily because they did not do so and compare headphones 'as they are'.
A bit like trying different speakers in an untreated room with lots of nasty reflections. Sure some speakers will sound better and a few may even sound 'good' but won't ever become 'excellent' without room treatment/DSP.
My advise would be: listen to headphones with familiar music on your own gear if possible and find one that sounds good and has a good comfort and fit and is suited for the circumstances you are going to use them in (travelling, home, office, gaming, outdoor, indoor) and has the right functionality (BT/wired/portable/NC for instance).
The more established brands usually provide pads and headbands or other parts for much longer than the cheaper or exotic brands.
More expensive does NOT equal 'better' but could be better.
There are gems in most brands out there at affordable prices.
Don't be afraid to use EQ.
Don't dismiss a headphone directly when comparing them, you may miss out on a great product once your brain has adjusted after hearing a 'hyped sounding' headphone.