I am currently using FR matching to EQ my DT-770, and think its workin pretty well so far.
If you’re going to try to match the FR of one headphone on another, ideally you’d probably want to use plots made on the same rig, by the same grapher for the best consistency. And also try to mitigate as much loss in sound quality as possible, using some of the tips here…
I still have some doubts about whether this would really work in the way some might hope or want though. And I’m not sure you can really turn a proverbial sows ear into a silk purse with this method, mainly due to the limitations of EQ and headphone design.
Fwiw, here’s what the difference between the HD800S and HD6XX looks like using measurements made on two different HBK 5128 rigs, by different graphers…
The 800S appears to be more forward in the midrange than the 6XX. More depressed in the upper mids/low treble. And more sibilant around 6k, based on these two graphs.
I’d also remove the narrow band peaks and valleys with smoothing before using something like the above as an EQ/correction curve.