For those that read German or can use Google Translate, here's an interesting study on the topic: http://www2.ak.tu-berlin.de/~akgroup/ak_pub/abschlussarbeiten/2010/RotterAndreas_MagA.pdf
It certainly has it's limitations, but it's interesting. Too bad headphones all have different housings! Lol. Anyway, if the last comment was directed at me, I was unaware that there is an antiEQ faction in headphone listening. I knew there used to be a long time ago in HiFi loudspeakers. I certainly didn't mean to offend someone or anyway. It's about the love of music, sound and gear.
The trumpet and trombone analogy works pretty wellin this scenario--you can't make them equal. You might be able to take an annoying, ear splitting tone out of them with a mute though.