Thanks for your response! I've read parts of that paper now and it's interesting, albeit only concerned with FR. I'm aware that different ears have different signal modulations and that paper shows this quite well (although I wish the plots were digital color images to see individual traces). However, "finding the perfect tune" isn't my concern here.

>Frequency response is the only measurement that matters for headphones (CSD and waterfall are useless).

This is much more what I'm after -- why are they useless in your opinion?

>I recommend trying two separate headphones and EQing them to the same target and you'll experience that they sound differently in areas other than tonal balance.

Yes, that's my starting point for this. Precisely because there is more to a headphone than timbre, I want to know how these things can be measured. RTINGS measures a bunch of things (as listed in the first post), although none of that relates to dynamics, and a few things aren't perfectly clear to me, plus I don't know how agreed upon their methods are in the headphone world.

BTW my comparison was between the 990s and 1990s, which have very comparable timbre, but the 1990s have what I would describe as a larger dynamic range, faster response, better imaging, and cleaner sound. Or the other way around, the 990s sound a little lush and sluggish in comparison.