It could be, but I’ve tried other headphones with drivers parallel to the ear, with potentially stronger magnets, and similar shallowness of pads that had stronger bass and caused no fatigue. But I do think it is related to the design of the 6xx series housing.
I do think increasing the bass volume and extension on the HD 6xx series housing just doesn’t work for me, something about it’s design and stronger bass doesn’t work to my ears. I found the HD 650 a bit unpleasant too in the bass and that may be why.