When you own quite a few iems like I do you will realize that burning in is a crock.
This is why I believe so. Excuse the poor narrative.
I begin listening to my RE0 for 20 minutes. Ahhh love this sound. Very transparent sound, lots of clarity, bass is nice.
Pop in my IE8s. What the hell?! Muddy, flabby bass. Where has my mids gone!?!? Dude, where's my mid?
20 Mins later...
Wooaaaahhhh super freaky bass...heyyyy, there's my midrange.
Put my RE0's back on.
What the hell?! Sound is so damn thin and grainy...where's my bass?!!?!?
20 Mins later. Ohhhh......here we go. Welcome back sexy RE0 transparency.
Pop in my TripleFi 10s.
Arrrghhhhh....Treble is piercing my ears. So painful and very metallic sounding.
20 Minutes later.
Woah no longer sounds metallic. Treble sparkle is awesome. Really punchy bass. Lots of clarity.
Pop back IE8s in.
Oh no not again.....muddly, flabby bass. Midrange MIA.
This is why I believe 'burning in' is a complete crock. From my experiences like this it is all in your head.