You seem to be begging the question as to whether a large soundstage constitutes distortion. In fact the opposite is probably the case.
Soundstage is just stereo imaging and in normal hearing is based on differences in time, including phase, and amplitude between the two ears. if these do not exist you get monaural sound. I can't see any way in which headphones could create a larger soundstage than is determined by the time/phase/amplitude differences in the recording. Thus the wider the soundstage the more accurate the reproduction is with respect to stereo. If the soundstage is compressed this will mean that some form of crosstalk is occurring. ie. the two channels are bleeding into each other either electrically or mechanically and that means distortion.
In other words, a narrow soundstage due to headphone crosstalk is just as much a sign of poor performance in headphones as it is in an amplifier, or digital source.
Keith Mitchell has reported measurements of this crosstalk
See p26 of this
http://www.politicalavenue.com/108642/US-MAGAZINES/Hi-Fi%20News%20-%20July%202016.pdf in HiFi News and Record Review. which he contends is coming through the headband. I.e the vibrations from one earcup pass along the headband to the opposite earcup. And by vibrations were are not talking about airborne vibrations so much as Newton's "equal and opposite energy" going from the driver into the surrounding material, thence to the headband and then to the opposite earcup.
I know of at least 4 companies trying to get rid of this problem, Sennheiser being one. The HD800 has damping material specifically in the headband and that is probably why it is reported to have a wider soundstage. Senn doesn't say what this material is only calling it "spaceage material." I would imagine they are also doing this in their $50,000 electrostatic as well.
Audioquest and B&W specifically set out to eliminate this crosstalk in some models and Grado employs a "proprietary polycarbonate" material in the conatruction of its e-series phones. They refer to this as helping transient response, but to the extent that it dampens the Newtonian vibartions, it will also reduce cross-talk.
Som years ago I got involved in this issue when I discovered that you could dampen vibrations on Stax SR007's simply bu touching the headband. One thing led to another and I have spent a good deal of time looking into the use of materials like Sorbothane to get rid of mechanical vibrations. Done right it gives a big improvement to sound quality and I should add widens the soundstage. See also below.
http://www.head-fi.org/t/671314/stax-sr007-resonance-problems
http://www.head-fi.org/t/744839/damping-mechanical-energy-distortion-of-stax-and-other-phones-with-sorbothane-and-other-materials