In my personal quest to find the best possible sound (for me and for my headphones) the pads have always been my number one priority (cup lining and/or driver modding and possibly even diaphragm tuning [on the HP 1000] can wait).
I have compared my Joseph Grado flat pads extensively and repeatedly with all my other pads (TTVJ flats, bowls and HD414s) on all of my Grado headphones (HP 1, Joseph SR100, Joseph SR325, vintage RS-1, John SR200, SR325i). The Joe Grado flat always constituted a clear improvement and not a side step. And what they do positively to the sound of the HP 1000, they do it positively to the John Grados also.
The HP 2i isn't like a different HP 1000; there are very little difference between them. I've spent much time analysing pads and realizing which does what and contributes what to the sound, so that I can isolate the headphone better in my critical listening (i.e. so that I can subtract in my head the effect of the pad that I'm using and be assertive about the headphones purely – i.e. make comments about the headphones independent of the choice of pads); I still have to pinpoint/understand in which ways my HP 2 was improved.
:/
But taking the time to reverse engineer and understand what these little foam “ear-adapter” do to the sound has been immensely rewarding for me; my special tape-modded HD414 pads, in my opinion, best even the Joe Grado flat pads in resolution and bass, with the negligible bonus of being cheaper to buy, more accessible / readily available (though they take quite a bit of time and are tedious to make) and more comfortable.
The effect of pad configuration on Grado headphones is monumental and drastic. When I adjusted and reduced the “aperture” (reduction of the volume of the “tape air chamber” [between the headphone and your ear]) of my sunflower pads (that's how I'm going to call my mod) by a few millimeters the sound went from much open to much closed in, lost airiness and details in profit of bass and fullness – overall sound signature became significantly darker.
Without tape (or any other rigid material; I like 3M Scotch tape) to conduct it, the bass bounces only once on your head/ear/tympani and then escapes by the air bubble matrix of any given foamy earpad.