My point was that unless you are doing a blind test where you don't know which device you are listening to, any claims about relative differences could be the result of placebo, which tends to run rampant in the headphone community. People tend to "hear" that certain things sound a certain way if they are expecting them to, and soundstage is such a nebulous concept that it is already pretty difficult to quantify and compare.
As far as I know, perceived soundstage is dependent on the physical design of the driver enclosure and earpads (size, shape, pad material, driver size/shape/angle, open or closed back, etc.), as well as the actual frequency response of the driver. It seems like headphones with more treble emphasis tend to be said to have a "large soundstage" more than headphones with a large amount of bass. Therefore I could see an amp affecting the frequency response slightly which could have an effect on the perceived soundstage, but I would think it would be very minor since the major contribution to perceived soundstange seems to be from the headphone design itself.
For example, my K702 has a much larger soundstage than my SRH 840, but the drivers are also farther away from my ears and the pads are much larger on the inside on the AKGs, which seems to be the major contributor (in addition to the fact that they are open and the Shures are closed).