Update:
I listened to Hilary Gardner and Ehud Asherie's The Late Set on different headphones three times today, which is out of the ordinary for me. The first time was on the new Sennheiser HD 599 SE, the second was on the Alessandro MS-1, and the last was on the Grado HP-2.
The HP-2 is clearly more resolving, and it has an excellent tonal balance throughout the audible frequency range. All three headphones are relatively neutral, but the HP-2 with the Deluxe pads always takes the crown with good recordings. In this case, I'd like to reiterate that the improved highs and lows are very subtle – on the Joe Grado headphones, you're not going to suddenly get bright treble and booming bass. The new pads fix the weaknesses of the headphones, but in such a complementary way that you wouldn't notice anything if these were the only pads you'd ever used.
In particular, you can hear Hilary Gardner's breathing more clearly, I think I can catch Ehud humming quietly while he's playing piano on a few passages, and you can more palpably feel the body of the piano rather than just the notes (like on the other headphones). The recording setup seems relatively simple on this recording, so there's no huge soundstage or production tricks to speak of, just great performances and great music. Gardner's singing is impeccable on these classic gems – she imparts a bit of playfulness on the fast tunes and seems to effortlessly breeze through some challenging songs while giving them the emotional depth they deserve.