Out of the stuff I know:
SR-007 Mk1: reasonably neutral and flat through the mids, the deep bass is down a few decibels and upper mid/treble is down a few more decibels save for a narrow spike at 10khz. Overall they're somewhat warmish with highs that are soft and sparkly at the same time. Not quite neutral, but pretty close to neutral.
SR-007 Mk2 (SZ2 serials): kinda like the Mk1, but with more upper midrange and midbass, and less treble extension. Less neutral, but still reasonably accurate.
K1000: I haven't spent
too much time with it but it's got some broad emphasis in the highs and deep bass is MIA, otherwise it's relatively linear.
Qualia 010: There's something strange going on in the mids there, the midrange tone is noticeably wrong. Otherwise, a fairly well-balanced and even-handed headphone.
On paper at least, the 009 looks to be the most neutral of them all, but I haven't seen measurements for everything (which you can find
here). The 007 is pretty close to neutral too.
If you want totally flat, flat speakers in an acoustically-treated room are your only choice.