Now you are comparing open vs closed which is totally unfair as you will have to work hard to get an open headphone have a smaller soundstage with more resonances then truly closed.
As for closed headphones jvc dx 1000 do pretty good. Guess how it does it. Teletubby cups to put drivers further away. It works
Well, if great soundstage is a required feature that someone wants, then a closed set of headphones is not where you try to find it..
And yes, if you mod a closed set or an open set by putting on big thick pads, then you have created a larger sonic "room" between your ears and the driver, so, you'll perceive more soundstage, but you lose something in the FR, in return.
One of the ways that Headphone makers try to get you to buy their closed sets, is they tell you that "its almost like a open back".
No its not.
Its a closed back and its going to have an intimate presentation, as compared to the open version.
The Physics of Sound, don't ever change, while the marketing
hype is ever evolving.
And truth be told... if a Maker is trying to sell a new "closed" gear by talking about "its soundstage is almost like an open back", then that is because they cant sell it based on its GREAT SOUND< or they would be talking about that, instead of soundstage perception.
Some of the ZMF's that are closed, are reasonably wide, regarding soundstage.
The original HE1000V2, is where you want to put your money, if you want a good sound and a "immersive sonic experience".
The HD800S is "wider", but, you lose a lot of low end.
Sennheiser makes an expensive Dac/Amp that puts it back and makes the high end of the HD800S more polite yet still very extended.
If you love closed sets, if you prefer them, then sometimes running them balanced can give you a tad more L<>R.