Burn in seems to just "bring it all together" for me. Doesn't change the sound per se. Just blends all the parts of the stew. Headphones can sound a bit stiff out of the box. These are flexible membrane drivers after all. Need a little running in.
Shure SRH840 serviced me nicely in early times of "discovery". They are my reference for all intents and purposes. I just play Van Halen's "Push Comes To Shove" and all becomes clear and they are the yardstick.
Metalhead here. +1 SRH840. Original Grado SR80. Senn HD485. I like AKG K240 Studio as well but those love juice so I use a Fiio E17 with it and my ipod classic. Metal is a tough genre for headphone love that's for sure.