By default pleathers have a bit more of a sub-bass rolloff compared to velours. My hypothesis is that since velours are thicker, they press against your head harder and so the contact between the earpad mounting ring and the headphone cup is more rigid, conducting the <30Hz subbass better.
However I described a modification that makes sub-bass extension very competent even with the shallow pleathers/jergpads earlier, it basically puts their sub-bass on a level field with velours or other thicker pads e.g. j$'s:
It works extremely well, on my HE500s the 20-24 Hz region which was previously flabby and without any air movement is now very tight and strong. I'd say there's about 5 dB roll-off from 30Hz down to 22 Hz, then another 5 dB roll-off from 22 Hz to 20 Hz. Beforehand it was a cliff past 30 Hz.
My guess is a major contributor to Audeze's amazing bass extension is also this, the fact that the bass of the earpads are actually adhered to the headphone cups.
Edited by jerg - 2/22/13 at 6:15am