That site is like a best friend to me now WOW i have to get some Ananda's like right the F now.
I would take everything there with a grain of salt if I were you. I also initially saw all that theory and thought that they got the soundstage thing nailed down, but in practice this doesn't translate to the actual experience. I think they make too many untested assumptions and basing all the calculations off of those.
Anyway, in my experience there are several factors that contribute to wider sounding soundstage. First of all, drivers that are angled and located more in front of the ear actually sounds narrower. If you push the driver backwards, it makes more of the sound appear to come from the back of the ear, thus enlarging the stage. One of the reasons planars tend to sound wider is because the driver is so large that a lot of the sound hits the ear from the back. Also, drivers that are located farther away from the ear sound wider. As you put drivers more to the side and away from the ear, the stage increases. A simple trick to "widen" any headphone is to put thicker pads, and preferably straight ones rather than angled.
Another factor that I noticed is that more bass usually makes wider stage. I'm not sure why that is, but I think it has to do with how lower frequencies are less directional and the mind thinking that if it hears more bass, it means it's farther away from the source of the sound. You can try using EQ to give a gentle tilt towards the lower frequencies. It works best with the very lowest part of the spectrum, around 20-100 hz. Boosting above that just creates mud.
Openness does contribute to wideness, but as proven by the 600 series from sennheiser and Stax headphones, it's actually less important than the two previous factors. Having the driver far away from the ear in an open design will do more to wideness than a close driver. But having a closed design or just not very open will diminish the effect of a far away driver
In the pursuit of the widest soundstage, there is an important tradeoff to consider - You always trade soundstage for imaging. The wider the soundstage in headphones, the less authentic it is. Basically, the hard stereo separation in headphones is not very realistic, so the wide soundstage is a side effect of the magnified stereo effect. That means that sounds that were supposed to be heard by both ears can sometimes be only heard by one, thus ruining the image illusion that would have existed if you were listening with speakers. I think that's why the HD800 is so successful here – the drivers are far away from the ear and with a super open design, angled forward, with an accentuated treble and almost no resonances or interactions with the cups. All of those factors combine nicely to give a wide sound that also images great. The drivers themselves are good but not too special, and it's the acoustic design that makes that concept work.