Is the housing the only reason makes the sound stage so big? If that is the only reason why don't other companies do the same? Even DIY people can try to 3D print and come up with more than half decent result. Not bashing you guys but I wonder why no one has done such things. I like big sound stage and want to know more about it.
I've pieced together a few rules of thumb from DIY experiments over the years.
The basic
shape of the soundstage is often determined by FR and can usually be modified to a degree by EQ. IMO a DF EQ usually give the most natural soundstage with more forward projection. Reducing DF's 2.7kHz peak and/or shifting its frequency can adjust the depth and width to a degree.
Overall "openness" is another factor. This is kind of akin to cabinet resonances in speakers. An open design can just let the backwave and any other excess energy radiate away. Closed designs need to damp and absorb it.
IMO, the biggest factor in soundstage is the positioning of the driver with respect to the pinna. When the driver is is larger, farther away, or at an angle with respect to your pinna it adds more of your personal pinna transfer function to the final sound reaching your eardrum. This makes it sound just a bit more natural and will increase the size of the soundstage.
Putting these all together we can examine a couple designs.
The HD800 has a larger than average driver held at an angle and distance for the pinna in a very open design. This gives a greatly enhanced sense of space. The downside of holding the driver in that position is the less than ideal FR with a peak corresponding to the wavelength of the diameter of its cups.
In contrast the HD650 has a smaller driver pressed directly against the pinna. This makes for a much more controlled "room" for the driver but greatly reduces the sense of space.
Electrostatics are also known for their large soundstages.
Compared to dynamics they have much larger drivers. They are also very open. Their baffles are sealed (or ported) on the ear side, but the drivers themselves very acoustically transparent. Some planar magnetics can nearly match a 'stat in driver size but the drivers themselves are less acoustically transparent. This can be easily demonstrated by just holding your hand an inch or two out from the back of headphone and hearing how much the sound changes. No current production 'stats I'm away of though have angled drivers. There is the old Stax Sigma though.
Overall, this makes a very open sound with a great sense of space. The non-angled drivers drivers keep it from having much in the way of forward projection though.
As to why this kind of thing is copied more? I'm not entirely sure.
Part of it is probably difficulty in getting a good FR in a physical design like the HD800. The HD8XX is now their third try and the prototype still needs just as much EQ as the original.
Part of it is probably cost. The other path to this is large drivers and those are expensive if you want them to sound good.
'Stat drivers aren't inherently expensive, but the amps they require are, and that is a hard market to break into.
Large planar drivers are expensive too. Lots of strong magnets trying to to tear apart the stators, lower yields on larger diaphragms, probably more issues tensioning them, and then the issues in making them acoustically transparent.
If you make a dynamic driver too big it stops being a tweeter and turns into a midrange. There are plenty of 40 and 50mm headphone drivers which are already twice as large as average tweeters on speakers, but which are expected to got up to 20khz on their own none the less. That's probably why Sennheiser went to the trouble of making the HD800's 56mm driver a ring radiator.
...
Anyway, these are just some random thoughts before I've even finished my first cup tea this morning...