my quick and dirty thoughts (after a very not-so-quick comparison) - my main headphones are the HE400, RS1i and ATHM50. I pulled out 12 favorite test tracks representing all the major subgenres of metal. I did use crossfeed, because i don't listen to my cans without unless i'm on the go out of the ipod.
HE400 - fantastic bass, especially quality. nothing one-note here, complex bass lines and all manner of kick drums are clear, separated and there is no bass bloom into mid bass/midrange at all, however there is plenty of punch and a really nice fat low end to the overall sound
midrange is recessed somewhat, which can really take away from a lot of metal vocals where the growls are lower than normal vocals. still crystal clear and excellent separation, any different guitars playing are easily separated. there's something about the midrange tone that i find slightly off though, almost a dark timbre. but it's very smooth, no bad habits
treble is sparkly (some would say hot) and all cymbals and solos are clear, separated and very smooth. soundstage is excellent, not too large or too closed in, but kind of far away sounding with the recessed midrange. The "speed" is excellent, never a moment where it feels congested or bogged down
RS1i - the bass doesn't reach as low as the HE400, and there is a definite lack of the fat low end (dark) feel that adds weight to some tracks. However...the bass is very high quality, nothing one-note here either, very well controlled and classic grado impact on kicks and double bass. No bloom into the midrange at all, the midbass is definitely emphasized but with most of my test tracks that's a really good thing.
midrange is just amazing, i think of it as liquid it's so smooth....however it's the exact opposite of the 400's, right up in your face. To me, for all my test tracks this is where the midrange should be, considering the majority of metal happens here in the midrange. Vocals are excellent, death growls all up in your grill. The best part though....the amazing tone/timbre. This is, to me, what metal guitars sound like. Instrument separation is very good, but not up to the he400 level, and although to me there is never any loss of detail, it can sound closed in or even slightly congested at times
treble is classic grado, some think it's hot but i don't. smoother than the 400's...smoother than most cans i've ever heard. like the midrange, the tone is excellent up here although also like the midrange it can sound a bit more closed in...again i think the detail is never compromised but separation isn't quite up to par with the 400's. soundstage is front row, and the "speed" is on par with the HE400, there is never a time where i feel they are getting muddy
ATHM50 - doing a head to head with these cans actually surprised me a lot. The bass is excellent in quantity, the fat low end is almost equal to the HE400. However...the quality isn't even close to the 400's or the RS1i, it's not as one-note as say beats but it's surely harder to separate complex bass lines and although kicks/double bass has a ton of punch, they are easily not as clear sounding, more rumbly. There is noticeable bloom into the lower midrange, although it kind of acts as a midbass bump like the RS1 you can tell it's from a bloom rather than an actual controlled bump
midrange is somewhat recessed but not as bad as the 400, the tone is in between the 400 and the RS1, this is where i was surprised is how similar the midrange sounds to my SR80, just less in your face. overall impressive guitar sounds, although it can get muddy when the song gets really crazy.
treble is peaky, the detail isn't bad at all but nothing smooth here. cymbals and solos are represented not bad but the separation is nothing like either of the other headphones, again there is a certain muddy quality probably from the peaky/notchy nature of the treble. soundstage is awful on the M50 as many here will know. the speed is average, kind of like the HD5*** series from Senn...too laid back and sometimes congested and/or muddy
Overall - I'm a personal fan of the RS1i. it's my favorite metal headphone i've ever owned or demoed or heard. I can understand that it's probably a personal thing though, but for me the strengths are just a lot more than the weaknesses. If i was recommending to others...the planars are a better choice, super fast with excellent bass and a much better all-around can for different genres. The HE500 is a better choice for metal, it's got a much better midrange but I personally prefer the HE400 (i use mine mostly for electronic and hip-hop). I've demoed the LCD2 a few times now, and I prefer it to the 500 or the 400....i just can't afford it and honestly it doesn't beat the RS1 for me, for metal. The M50 surprised me, when doing a side by side it actually sounds better than I always convince myself that it does. Maybe i don't need a new closed can as bad as i though LOL