I've now compared my
Drop Sennheiser HD6XX vs Sennheiser HD800S, both with the corresponding
Oratory EQ filters applied!
With both EQ'ed, they do sound quite similar in overall tonal character, but there are definitely noticeable differences, and the HD800S still wins my preference by a good margin! There's a large soundstage, clarity, and 'openness' improvement from the HD800S, as well as being the most comfortable headphones I've ever worn.
I suppose it's possible that the frequency response of the HD6XX does not match the HD650 close enough (I think they're supposed to be the same headphones just rebranded, though) for the Oratory EQ filters to work at their best. However, the fact that the appropriately EQ'ed sound signatures of each headphone sound very similar seem to suggest they're at least partially applicable to the Drop.com Sennheiser HD6XX.
I'm not sure what explains why the HD800S sound so much better, but perhaps they have lower IMD or something like that? Or perhaps the location and angle of the drivers relative to the ear? In any case, the results are fantastic and I will be keeping the HD800S as my new primary headphones
The HD650/HD6XX have been my only tolerable long-term listening headphones for over 10 years since I discovered them, and only now thanks to the very effective EQ filters I can replace them with the HD800S (which otherwise are painfully bright, so much as to fatigue my ears almost instantly).
The HD800S equalized is definitely the best sound quality I've ever heard from a headphone, and comes quite close to the overall experience of some of the best speakers (like my Genelec 8351B's) minus the tactile fully-body bass sensation of course, and is surprisingly speaker-like in its soundstage (though of course a headphone can never perfectly match that aspect of actual speakers).