Mine were good new. Haven't changed in weeks of use. My sound perception changed as I got accustomed to the new sound they present, but not the headphones.
Burn in is psychological - the sound gets better with a good new component as you supplant the old memories with the fresh memory of the new component - takes some time to really learn the sound of the new.
If someone you know buys a new K701, have them do switching where your eyes are covered, recorded your response as to which sounds best. The new pair will sound best just as often as the old burned-in pair - you won't be able to tell the difference.
However, be sure you face magnetic north as you listen for the first 500 hours.
This allows the new magnets in the headphone to properly orient the north and south poles of the earth with use.