When it comes to soundstage I think we're all on our own to some extent. For me the only thing I've learned is that spacious earcups that don't touch your ear combined with angled pads or angled drivers is what seperates headphones that have good soundstage vs bad soundstage - assuming you've EQ'd each headphone to the Harman Headphone Curve (because measured frequency response on a dummy head does affect the soundstage too). RTings tried to once quantify the potential soundstage of a headphone by characterising it's "pinna activation", which they based on the premise that the HD800 was the King of Soundstage, lol! No, but that's a fair assumption I guess, but I don't know how they actually quantified that variable, and in the exposure I've had to RTings I don't think they characterised that variable that well. It's just still an unknown, but me personally I stand by my observations re spacious earcups / angled pads and/or angled drivers.
My own experience, I have to say that my K702 (I have 3 units), is my best headphone for soundstage, and that's after EQ to the Harman Curve. .........
So yes, there's no real measurements associated with soundstage - it's a distilled property from personal and other's observations, there's been some crossover & agreement in what physical aspects of a headphone create good soundstage if I look around here on ASR and other places.
There may be something other than angled drivers that affects soundstage. Dan Clark claims to improve sound-staging with earpads and FR, and that may be at least partly true.
When I was doing the headphone comparisons (yes, Harman-tuned) I spent most of the time comparing the DCA Closed-X to the Noire. With soundstage, I was curious to see if DCA was able to achieve superior soundstage with the Noire (as claimed) without using angled drivers. Dan Clark said the frontally-perforated earpads (introduced on the Closed X/RT) and FR tuning (more U shaped, less mids) were the tools.
Out-of-the-box, the Closed X had the earpads, but not the tuning. I was not impressed with the soundstage. It was almost always linear from ear-to-ear, never further "outside" than the middle of my forehead, even with nicely-recorded selections. I really like the Closed-X, but I am sensitized to the ear-to-ear thing and dislike it. The pads alone don't do it for me.
The Noire is better. I never get the ear-to-ear staging that I dislike. It is most-often not a large soundstage, but it is almost always apparently outside my head, 9-3 o'clock, and does give the impression of being in the audience, with a balanced spread of instruments. With good recordings like jazz ensembles, or even things like PJ Harvey(!) I could get a very deep soundstage when the recording venue also had one and the recording was well-done, albeit as an exception rather than the rule.
I've spent a lot of time on FR impact and simply re-discovered the true benefit of a U-shaped EQ to soundstaging. Out-of-the-box, the Closed X has a more mids-forward FR balance, and that does benefit some music (like close-miking can appeal on some nice vocals). The Noire does better on recordings that require tonal balance and a wide-soundstage, such as (but not only) orchestral music.
On my Noire, I now have two EQ's. One has Oratory-like mids, the other is 2-3 dB more focused on the mid's. I thought I could toggle back and forth based on the material. I spent a lot of time optimizing the two choices. But strangely-enough, on the fly, I almost always prefer the version with more recessed mids, even for material I analytically thought was more exciting with a mid's focus.
You'd think I could reproduce Noire-like soundstaging on the Closed X by tuning it like the Noire. I did try that, but it didn't work. I don't know what the hidden variable is with that.
Anyway, I do prefer the Noire for soundstage, and unless Dan Clark has another secret ingredient in his barbecue sauce, it seems to be due mostly to tonal balance. If the front-perforated earpads also provide a bit of the benefit of angled drivers, then they may be a help to soundstage, but not sufficient by themselves to do it. The combination doesn't turn the closed-back Noire into the HD820, but it makes a real difference to me.
As to the examples of deep-soundstage that I sometimes get with the Noire, without an alternative explanation, I tend to ascribe that more to perceptual cues in the recording (like volume and tonal balance of voices, reverb, etc.), rather than the 'phones themselves. But when it works, it works.