I find this is a very interesting psychological question. I am sure that we can perceive sound signature changes in critical listening with new headphones, but are these changes real, or is it just that our brains are adapting to the sound of the new headphones. Moreover, if sound quality improvements are expected during burn-in, most likely changes will also be heard, whether it happens or not. We humans are not that good at "referencing" sounds we hear. Therefore it is also very difficult, or even impossible, to dispute burn-in.
Actually, when I decided to purchase my new K701s over a week ago, I contacted AKG via their email-hotline. I wanted to know what their thoughts on burn-in were before I would begin listening to the changes first hand. Couple of quotes from AKGs response:
"From our experience and knowledge we cannot confirm that there is a burn in effect of the transducers taking place."
"However, during the first hours of use of headphones, the ear pads - in the
beginning a little stiff – start to accommodate to the users ears and head
and the sealing becomes better, as a result the bass can be increased a
little, on the other hand the distance between the headphones and the ear
may become closer, i.e. fewer air volume between ears and headphones is
available and thus less bass."
I have also ordered Alessandro MS-2s from their website (still waiting for them). So I also contacted Grado and asked the very same question:
[size=x-small][size=x-small]"there is NO scientific evidence that a prolonged "run-in" is beneficial. [/size][/size][size=x-small][size=x-small]In fact we don't recommend it[/size][/size]. [size=x-small][size=x-small]We recommend that one uses there headphones normally, there will a "loosening" of the suspension after a period of time and it will settle in ( the material do follow Hook's law)[/size][/size][size=x-small][size=x-small]. those who run in their headphones for a prolong period of time risk damaging them. [/size][/size][size=x-small][size=x-small]after a "workout" the drivers must rest, just like going to the gym you have to rest your muscles.[/size][/size]"
I personally do not believe in physical burn-in. Transducers do their thing millions of times in a of couple of hours. I have not heard any changes with my K701. I have about 25-30 hours on the headphones. This of course does not prove that there are no changes. Someone who has better ears than me could possibly hear them, no doubt. I don't think there is right or answer on this one. I am eagerly waiting for the MS-2s to arrive and I will again carefully listen if I can hear any burn-in.