I have 2 EP K400's with the rubber headband, same driver, and both sound different! <(x_____X< ) My first K400 was in veeery good shape, looked almost like new. The only hint, that it gave me were the pads. They were extremely used and smoooth / worn-out. My second pair was even in better shape, almost new. BUT . . . this time . . . even the pads were like new. Stiff and not so smooth.
The older one sounds warmer and more organic. The newer, unused one sounds colder and . . . more like a K401 / K500, kinda more analytical. I think, maybe it needs to be burned in (really like new, haven't gotten much playtime i guess) OR . . . it's because of the pads! I haven't had the time, to switch the pads and test out, if the older one sounds like the new one and vice versa. I really think it's because of the pads, because they even feel so different.
That said, to all K400 / 401 / 500 / 501 owners: Dont switch out your old pads, because they're worn out. They sound good as they are, new pads will affect the sound . . . to the negative ~ at least for me. But maybe for others it's the other way around and they favor the colder sound with the new pads.
Edited by Natsukawa - 8/12/13 at 4:29pm