Let me fix that for you ^_^.
Your experience of a new headphone relies mostly on your experience of whatever you were using before that for a long period. As you spend more time listening to the new headphone, your brain adapts to it(trying to work out what the new normal is). So your assumption that your preferred response or even your impressions didn't change is
very likely to be wrong. Do not underestimate the adaptability of a brain!
Then we have very objective, very acoustic related possibilities of signature changes. The pads slowly conforming to your head, or a different adjustment in the way you wear the headphones. That could easily get you one or 2dB deviations.
Here is the old graph from Innerfidelity with the grey lines being the measurements of 5 different placements on the dummy head.
So clearly I'm not claiming that it's all in your head. I'm simply making a list of possibilities, and out of them all(I might be missing some, like, did you drop the headphone since day one? Was there a massive change in temperature and humidity over that period of time? Did you clean your ears? etc), I wonder how you decided that your impressions had to be caused by driver burn in?
It's always good to make a list of possibilities, and to rule out only the ones we actually can rule out(wanting to, doesn't count
).
I learned on this(range at the bottom):
I don't know if that chart should be a reference. It's been mine for a long time, so if I talk about midrange, you can be confident that I'm referring to this. ^_^
About flat midrange, I'm sorry to tell you that with headphones, there is not much that applies to all listeners. A graph a dummy head measures is based on some averaging of human heads. But beside a bunch of guys falling right on that average body, everybody else will be a little different and a measurement at their eardrum(if it was possible while alive and not screaming from pain...), would show a different frequency response.
For example, Harman came up with a headphone target(very serious work) that seems to please a majority of listeners. So it's probably a good idea to start with something similar to that target. But a majority in this case is somewhere around 60%. Leaving about 4 out of 10 listeners wanting for something different!!!!
That doesn't help you at all, sorry. But hopefully it can help you avoid a few bandwagon traps later on. Beside trying to see if you get lucky with some standards, your best bet is to try a bunch of headphones with known FR, and learn to correlate how you feel with those graphs so you can determine where to go next. Or to play around with an EQ, also to learn more about your very own needs. I hate it, but some serious amount of luck is involved in finding our neutral or our preferred sound(often enough the same thing).