One, with the appropriate equipment and laboratory, should be able to debunk this or prove it conclusively by using a brand new set of AKG K701s, then testing them again after 400 hours of "burn-in".
Text from this excellent review from Stereophile August 2006:
Stereophile Review - AKG K701
"About a week later, I checked in on the K 701s and that edginess was gone, replaced by balanced sound with a natural top-end sparkle and a ridiculously robust bottom end. What causes such a change? Some folks speculate that the diaphragm becomes more supple with play, or that the motor mechanism wears in. I don't pretend to know what goes on, only that a few hundred hours of vigorous play transformed the K 701s."
If this is the case then a graph of the frequency response should unequivocally show proof, or the testing methods become suspect... I have read, with interest, the testing methods at Headroom, and see no flaws in their testing methodology, which is why I posed the question, especially with regards to the K701.
Definitely an interesting subject that I wouldn't want to see treated as an urban myth but rather as a question crying out for a verifiable and repeatable experiment.
Cheers.
Tony (Ottawa)