Ok, I was able to give a few hours of listen to various music A/B with ESW9 and M50. I am not good with describing music... I can't tell you if there is a drop off below * khz or what not... but I will try to describe how it sounds to me.
Listening environment.
FiiO e7/e9 combo
Music file, 256kbps amazon download to lossless ripped from CD.
Music
Beyonce - I am... Sasha Fierce
Rhianna - Good Girl Gone Bad_Reloaded
Vocaloids - various
Girls Generation - various
Various Chinese songs (more ballad type)
I think in general, if you like songs that are driven by rhytmic bass, the M50 is definitely better. It is more energetic and the constant bass rhythm and the interaction and other instrument create separation and energy. It sounds electronic, synthesized... much more club/dance feel.
For Rhianna, I A/B Umbrella many times, it is much more fun listening with the M50. Most of the sounds from Girl's Generation and Vocaloid stuff also sound better with M50. Strong bass, I feel like I am implored to move with the rhythm. Vocals however are less impressive. Details are there, but definitely take a back seat to the bass and feel rather unemotional.
ESW9 feels like a complete opposite to M50. If you want to hear nice vocal, this is definitely the headphone of choice between the two. The mids are sweet and emotional. Everything is very musical, but the rhythm has to be driven by the vocal and not the bass beats. I must've listen to Beyonce's Single Ladies 5 times, each time I prefer ESW9. I guess because she also oh oh oh the rhythm and I much prefer listenting to that the had that part purely carried by the bass. The Chinese Ballad type song and also slower vocal japanese songs are almost unversally sound better on the ESW9. I don't know how to describe the mid/vocal on the esw9... it is sweet, buttery and utterly enjoyable. It just isn't a good fit for electronic/dance music. Example of songs I enjoy on ESW9 is http://www.youtube.com/watch?v=FdLg-vLtiu0
I think for casual/fatigue free listening. I would think ESW9 would be choice. It is more colored, but more musical and bring things together better imo. The M50 just seem to match the electronic sound that it is a bit too in your face when a new instrument is introduced. But of course, I think the type of music that should be listened to on these 2 phones are completely different. Listenting to ballad/vocal songs on M50 I feel there is a distinct lack of emotion. All the technical parts are there, but nothing.
Anyway, I am not good at describing music. I am not a music major nor do I play any instrument... hell, I barely passed Music Appreciation back in college
Feel free to ask for any clarification tho, I will try my best to describe the differences in these 2 headphone.