It seems like all your plots show broad, smooth looking ringing starting at about 1khz. I figured it was more the testing method than the headphone, and your plots are meant for higher frequencies than that.
But maybe what's happening is that the T50rp is a much cleaner driver to begin with than almost all other drivers, and the stock driver is actually fairly nicely damped, so all the added junk here, while not much compared to other phones, is coming from the cups, which sounds different than the driver ringing other phones have as well?