Ah, my mistake, should've grasped the context.
Check out the HE400, the midrange rise in THD is partially due to it being a planar with single-sided magnets. While the distortion in the HE400 is apparent if you look for it, it manifests as a mostly benign flavor in casual listening. Despite its ghastly looking graph the 400 is a terrific sounding headphone, I have no qualms recommending it to others.
HE400 is NOT a single-ended ortho design. You're confusing it with HE4.