You all seem to be forgetting the infamous Stax Sigma. So far the only headphone that I ever heard with a real soundstage. Stax did it by essentially re creating a small room over each ear with the driver at 90 degrees up front. Arguably the ugliest headphone ever but truly amazing in the perception of space.
Not all of us can afford any Stax headphone. So having never heard one (or seen one in real life), I cannot bring it up.
But doesn't this come under the subject of driver angles? Aren't all headphones just a 'small room' around our ears?