False - the warranty has absolutely no conditions. You don't need any proof of purchase whatsoever. I am from Canada and purchased my pair used from a fellow Head-Fier from the States. I sent them back with a broken head-band and no cable (I removed the Stax retermination cable before shipping them back). They were sent back to the USA Koss address in a plain used shipping box with nothing more than my name, address and concern. Six weeks later, a brand new pair of ESP 950's complete with Koss cable landed on my Canadian doorstep. They ARE true to their warranty - NO conditions and NO questions asked.
Really? I don't know what gave that other Head-Fi poster that impression, then...
With a warranty like that, buying a used ESP/950 suddenly seems quite tempting, once I've got the cash.
Speaking of that headband, how comfortable do you find it? I get the feeling that the big reason I find the Lambda so comfortable is its headband design.