Oh I'm quite sure people would be 'modding' the crap out of their hd650s and akg701s if they were as easily switchable pads as the hifimans. Hd650 has an advantage-- or disadvantage, in that it changes sound signature much more drasticly than the purely resistive he-400. 'Amp rolling, is such a big deal with the sennheiser phones because everybody is trying to find their preferred sound with them and they know they're responsive to upstream equipment. That's more of a commitment than cutting holes in 20 dollar pads.
Really interesting point there. I would consider use of amps/DACs "modding" sound to get their personal favourite sound.