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As mentioned, an easy way to make the DT150 a neutral HP is to replace the pads with the DT100 velour pads. Bass response will vary between individuals anyway because of varying amount of lealage, depending on head size, fit, clamping force etc
I know this review was a long time ago but if you would have the DT100 velour pads and the DT150 still with you, it would be very appreciated if you could measure it with those pads on the GRAS. There is no other measruement available other than what @solderdude once made with his gear.