Some manufacturers of higher-end gear do burn-in (I believe zach at ZMF burns before shipping) and others (MrSpeakers) directly recommend some pretty big burn-in times of 2 solid weeks ... as the CEO directly told me when I got a set Aeon Flows (they're planar vs dynamic, but Zach's are dynamic). So there's two examples of high-end designers/owners recommending or performing burn-in. So if they think it's a thing, it's probably a thing.
Further, equipment like, say, the Schiit Yggy DAC needs to be on FOR A WEEK before it's up to prime SQ levels, so not only is burn-in a thing, but run time can be as well.
Those things being true, it MAY extend to lesser equipment like the PX suggesting that opening the box, firing it up, and playing music will not give you the true measure of the experience you'd get after a month of use and 2 hours of run time. Buuuttt ... personally, i've never worn a headphone pre burn-in that I've not liked, burned it in, and then liked it. That's not to say that I didn't hear a difference, I did, it just didn't change the SQ enough for me to change my mind.
Finally I'll say my burned-in P7W sounds stellar (albeit P7ey), but I've heard other P7Ws new that don't sound like mine and worse to my ear.
