It seems that many audiophiles have extensive rituals for burning in phones, from playing music continually through them for 24 hours, to blasting them with 40 hours of pink noise, to a large dose of Lou Reed's Metal Machine Music. Me - I just buy them and use them. I haven't noticed a difference between a well kept pair of older Sony 7506's or Fostex T20's (or my favorite cheap phones - the Monoprice 8323) and new ones unless the company upgraded the drivers. It all seems like a myth to me.
Shure has apparently looked into this phenomena as well and have found that if the phones haven't been abused, there's no evidence that the frequency response changes over time. Of course, if the phones are used in the studio where the musician has control over the level, chances of abuse are pretty good, as we all know too well.
Here's something else to think about on the subject. The competition in headphones in pretty fierce these days, so don't you think a manufacturer would do everything in their power to make their phones sound as good as possible right out of the box? If that meant burning them in for 400 hours before shipping, don't you think they'd do that?
To my knowledge, no headphone manufacturer uses a burn-in process yet, but if you know of one, let me know. I'm dying to hear the evidence that the practice is worth the time. If it is, I'll be getting out my pink noise generator.
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