After several tests with different pads (HD25 velour, HD25 pleather, ATH velour, ATH pleather) I came to the conclusion that the best sound to me comes out with the ATH pleather.
More confortable to my ears with good isolation. Balanced sound and more open soundstage. Pure bliss!
My only regret is having left the JVCHD25PP club,
Anyway, just like how I was astonished with how the memory foam tips on the FX40 change it's sound so much, I am equally astonished at how well the ATH leathers on the S500 changes the sound for me. Dsnuts, nobody can ever call you nuts lol! As you said, the mids are fuller, and the sound is beautifully balanced now. Full. Lush. Detailed. Still plenty of bass, but it just sounds amazing to me. Not fatiguing to me now at all. I can crank up the volume, and nothing is overpowering - everything is beautiful. Unreal!
So, I guess you could say that I'm a S500 + ATH leathers believer now, and the S500 actually is living up to the hype. I'm pretty damn happy camper right now. Thanks to dsnuts and doc and rick and all of you other crazy S500 fanatics - you done good!
Edit: Oh yeah, the ATH leathers really are quite a bit more comfortable - awesome added bonus :)