Some open-back HPs that are close to neutral...
OPEN-BACK - PART 1
It's pretty clear that the Sennheisers are king of the hill in this category, except that there are some planar-magnetics which are better extended in the bass. The Senns have a superior response in the midrange and treble though, and better flow than most planars imo. These are a few of the better ones imho...
RAW LEFT:
https://www.rtings.com/headphones/1-4/graph#891/4011
COMPENSATED L & R:
https://www.rtings.com/headphones/1-4/graph#891/3992
This particular plot of the left channel of the Senn HD 58x Jubilee has the best extension in the bass, and also excellent response in the treble. It's maybe just a tad forward in the upper-midrange. And perhaps also a tad dark in some spots in the treble, which is probably easier to see on the compensated plot. I expect to see a little bit of a downward tilt in the treble on Rtings' compensated plots though, on neutral HPs. So these are fairly minor criticisms. And resonances all line up well in the treble at approximately 3, 9 and 15 kHz. The bass extension on the right channel of this headphone is not quite as outstanding though.
Bass extension is a critical component in achieving a neutral response imo. (Though certainly not the only one.) The Senn with the next best extension in the bass (and decent treble response) in the Rtings plots is sort of a toss-up between three different headphones: the HD 650, HD 660S, and the Game One. We'll start with the 650 and work our way down...
RAW LEFT:
https://www.rtings.com/headphones/1-4/graph#245/4011
RAW RIGHT:
https://www.rtings.com/headphones/1-4/graph#245/4012
COMPENSATED L &R:
https://www.rtings.com/headphones/1-4/graph#245/3992
RAW LEFT:
https://www.rtings.com/headphones/1-4/graph#816/4011
RAW RIGHT:
https://www.rtings.com/headphones/1-4/graph#816/4012
COMPENSATED L & R:
https://www.rtings.com/headphones/1-4/graph#816/3992
The 660 has a slightly more pronounced dip between the lower and mid treble than the 650, which is fairly easy to see on the compensated plots. But very well-defined ear canal resonances at 3, 9 and 15 kHz. The 650 looks a bit more like the venerable HD 600 in it's low to mid treble range though, fwtw. Based on some comparisons
here, the HD 660S should have a noticeably darker sound in the treble than either the HD 600 or HD 650.
RAW LEFT:
https://www.rtings.com/headphones/1-4/graph#500/4011
RAW RIGHT:
https://www.rtings.com/headphones/1-4/graph#500/4012
COMPENSATED L & R:
https://www.rtings.com/headphones/1-4/graph#500/3992
The Game One does not have quite as well-defined a resonance at 15 kHz as the 650 and 660S. But it is otherwise pretty good in the treble.
RAW LEFT:
https://www.rtings.com/headphones/1-4/graph#325/4011
RAW RIGHT:
https://www.rtings.com/headphones/1-4/graph#325/4012
COMPENSATED L & R:
https://www.rtings.com/headphones/1-4/graph#325/3992
The HD 600 does not have quite as good extension in the bass as the above HPs. But it has excellent treble response. Which is probably a tad brighter than all of the others above.
Some approximate rankings of the above headphones in terms of their tonal characteristics, based on the above plots...
Treble Brightness:
1. HD 600 (brightest in treble)
https://www.rtings.com/headphones/1-4/graph#325/3992
2. HD 650
https://www.rtings.com/headphones/1-4/graph#245/3992
3. HD 58x Jubilee
https://www.rtings.com/headphones/1-4/graph#891/3992
4. HD 660S (darkest in treble)
https://www.rtings.com/headphones/1-4/graph#816/3992
The HD 650 may have a slightly darker tilt in the mid to upper treble than the HD 58x. But the 58x has a few more recessed areas in the treble than the HD 650. So depending on the specific areas you're looking at, it could be more or less a wash between those two.
Bass Extension:
1. HD 58x Jubilee (most extended in bass)
2. HD 650, HD 660S, Game One (all about the same)
3. HD 600 (least extended in bass)