Are you sure you measured them out of the box and some few hours later? When I bought a new pair of Beoplay H6 for my wife, it sounded more aggressive than mine which was already few months old. But, this wasn't true all headphones I came across. My 702's had very little or no change in sound from the beginning. But, my older AKG K551 was a different headphone after 5 hours of playing something in my wardrobe. I played the same song again, and I was not a believer of burn in then. So all I wanted was another proof that it doesnt exist, unfortunately I heard a difference, which I am sure was not from inside my head.
Yes, I usually measure my headphones directly from the box to verify channel matching. Then later multiple times for various reasons.
Check these measurements made by Tyll Hertsens from
Innerfidelity for the Q701 which is very close to the K702.
He has a high quality (and quite expensive) lab for measuring purposes.
0 Hours and 330 Hours frequency response overlapped for Left (top) and Right (bottom) channels.
Channels are spaced vertically to make them easier to read.
As you can see there's no frequency response variation from 0 Hours to 330 Hours.
Pad wear and headband wear along with clamp can make a difference on certain headphones. Seal is very important with closed back headphones and IEMs. Put them on without getting perfect seal, then place them properly and you'll get different sound. Placement can be of importance as well on various models including open back designs like the K702, more so if the drivers or the earpads are angled: move them and the sound will change a bit.
This is K702 measured by Tyll Hertsens at 5 different positions overlapped.
Check the corresponding grey lines. I've added some arrows. Look how different positions produce a different response.
This can play a role as well when it comes to perceiving a headphone as sounding different than before. It might be sounding different than before but not for 'burn in' reasons. According to most studies I've read, if there's some 'burn in' (logically nothing stays exactly the same eternally), its effects are very very subtle, not necessarily for good and come with thousand hours of use. Last but not least, do not overstate the human memory as it's not a tool for analysis. Our hearing can play us various tricks as well. My advice is to double check everything you seem to hear when possible. Try the same a different day when possible. Give it to someone else to try too. If it still looks real then start trying to find out where does it come from.
People normally want an easy "pick from the shelf" solution for everything.
I think it sounded different ---------> Burn in ---------> Solved
The real thing might be very different, result of multiple variables. Solving it in a proper way can be much more challenging, that's granted.
