I compared a number of cans relating to pleather vs velour (or a similar product): Beyer DT880/600, DT990pro, A900x and AD900x. In every case, and I mean every, the substitution of pleather for velour made the cans sound much more bassy, and negatively effected all other parts of the spectrum. In the case of A900x, where I substituted AD900x non-pleather pads, the cans lost their bass, significantly, as well as their sense of depth. The overall tonal balance became lighter. Essentially, the opposite of substituting pleather for velour. I got very similar results with the ATs and Hi-Fiman velour pads, though less drastic in magnitude.
Edit: text, spelling