Of course you are right about the subjectivity. (Kind of eats away from this site though ) I might find negatives later on, but it was quite apparent to me by my standards, which was the better one. I'm more confident I'll get used to the minor differences to my current reference, the K272.
3-4% might be about right for tonal issues. I wouldn't be able to comment on that, my hearing is not that good... Or I just listen to different things :)
The 8400, as I said, was the clear winner. Let me quote myself: "disregarding voicing preferences, tonal balance, FR and the such." Objectively, they simply were better at distinguishing notes, especially the brass and bass I got to hear from the Nucleus albums. With the 6400, I could not hear the bass groove nearly as well as with either my K272 or the 8400. The tonality was good and I had no problems with that. But it was weird how much they differed in the aforementioned aspect... For a musician who seeks to be able to tab notes, for instance, the 8400 just win hands down. I don't think I need to spend any more time with the 6400, since I don't care about sound itself, but the musical accuracy a headphone delivers. By this I simply mean: to distinguish variations in pitch. I've noticed that if a headphone does this right, it excels in many other parts as well.
I didn't pay attention to the forwardness or soundstaging of the mids, but the 8400 were cleaner and more clear, were it indeed guitar, drums, trumpet, saxophone or oboe. For simpler music, without as much melodic layers, tracks, harmonics and modality, the 6400 might do just as good and I'd be much more hard pressed to tell the difference. Put on some Zappa, Miles Davis or indeed Ian Carr's Nucleus (more groovy) and the difference in performance speaks for itself.
Anyways, can't wait to get them. They were highly comfortable as well. Very light, felt kind of cheap and hollow with the plastic, but not too breakable. Expected more from the memory foam, but it did the job and was at least better from the 6400 pads.
I think I got that impression too. Especially when taking them out of the box! They're holding up very,very well though.
With your comments, it does make a lot more sense now why you'd prefer the 8400. It's good to know at least that it MIGHT be able to compete with the K272HD.