Here's FR of an mod of one Russian guy. White is unpressed earpads, green - pressed.
Notable is flat FR above 10 kHz.
The mod is very extensive. He picks the drivers, removes stock paper, saws through metal plates, mades wooden earcups no less than 10mm thick, and does custom damping, cup volume matching and earpad matching for each driver till +-2 dB.
I still think that uber-damping is an easier way to achieve flat sound from Fostex. But to get flat FR above 10 kHz, one would have to damp within driver anyway..