I figured out how to increase the sub-30Hz bass extension (almost to the point of FR linearity between 30 and 20 Hz) with Hifiman headphones today, without affecting anything else.
This was after noticing that sub-30Hz sub-bass rolled off drastically the more I "propped up" the earpad mounting ring off the cup with tape stacks, which made me hypothesize that the lowest subbass is conducted from the drivers to our ears via the surface contact between the cup and the flat undersides of the earpad plastic mounting rings. Another inspiration was from the fact that all Audeze headphones had the earpads adhered to the cups, rather than just mounted on.
So I picked up a roll of double-sided tape (the kind that pretty much leaves just a thin layer of solid adhesive on whatever you want to bond) today from dollar store, and tried it out. Here's a rough diagram of what I did (excuse the shoddy MSpaint-job, I forgot to take photos of the procedure when I did it).
** note this is not the current version of Jergpads I'm sporting, the current version has much more extensive damping holes, as well as the pleather dust screen.
After doing this and carefully installing the pads and pressing down to make sure the double-sided tape is adhering the mounting rings to the cups properly, I did some listening tests.
Sinewave sweeps show VERY little roll-off under 30 Hz with this earpad adhesion modification on my HE500. Before, there was almost no real bass vibration (just a dry whomp whomp) at 20 Hz, now 20Hz is strong and robust, and very clean, just as well as 30 Hz tones. The rest of the frequency range seems about identical, as predicted.
As for music tests, I tried the album Mezzanine by Massive Attack, and the improved sub-bass immediately becomes apparent. Very Audeze-like, deep, "bottomless" sounding. Wow.
The subbass extension is likely on-par with LCD2/3 now.
I'd recommend this to those who have already done the Jergpad mod on their pleathers, and are satisfied with their modding job / results already. The double-sided tape I got is removable but obviously the adhesive won't be cleanly removed without some vigorous rubbing and scrubbing both on the mounting ring and on the headphone cup, especially after I let it sit for a few weeks/months/years.
So don't attempt it unless you are already completely satisfied with your pads and don't mind having them semi-permanently on the headphones. Then again, if you've already done the jergpad mod you probably abandoned the notion of trying to maintain the resale value of your cans
Edited by jerg - 2/19/13 at 12:05am