EricJ - I looked at the frequency chart at headphone.com. Based on my numerous listening tests, the frequency charts are very accurate representations of what I hear, so I totally trust them 100%.
I just did a 3-hour listening test at a local pro audio shop, and here's a mini review of the ones I tested:
ATH-M50 - By far the best of the bunch. Sounds very similar to my HD555. In fact, if it was open-back, it would probably sound just like the HD555. Nice and warm, lush, no harshness at all. No boosted or scooped frequencies, except the highest highs are just tiny bit rolled off for the warm sound (this is essentially for me, as most headphones are too sharp to me).
Equation RP21 - Very good for the price. Slight harsh at the high end, but everything else was great. The ear-cups don't fit as comfortably or securely as other headphone with full-sized ear-cups.
Beyerdynamics DT770 / DT880 - Sounded similar to each other, with the DT880 a bit more accurate. Both have that slight harshness in the highs that I don't like. I prefer the slightly warm Sennheiser sound. these are very comfy though--I wish the M50 and the RP21 had velour ear-cups like these.
Sony MDR-7509 / MDR-V600 - I have always disliked the Sony headphones due to the shrillness of the high end, and it's still the same this time. Absolutely couldn't stand it. Talk about listening fatigue!
Sennheiser HD280 Pro - It's ok. Nothing to write home about. At the same price, the RP21 sounds significantly better (although the HD280 doesn't have that slight harshness in the high end).
Sennheiser HD202 - for such a cheap price, it's quite good. Bass is hyped, and lower-mids are recessed.
M-Audio Q40 - Second best of the bunch by far. Almost like a middle ground between the M50 and the RP21--the harshness in the high end is almost gone--just a tiny hint that's barely there. The rest sounds remarkably similar to the M50. The ear-cups are a more secure fit, but some might prefer the looser feel of the M50 and the RP21.
Some Samson headphone - CRAP. Don't bother.
So, my final decision is:
RP21 (already bought a pair used from someone here on head-fi) - for the singer to wear during tracking.
M50 - for me to wear during tracking.
I could swap out the RP21 for the Q40, but since someone was selling the RP21 and no one was selling the Q40 used, I jumped on the RP21. I like the Q40 better, but I also like the fact that the RP21 will give me a different perspective as an alternate take on my mix, whereas the Q40 is so similar to the M50 and HD555 that I really don't need 3 very similar sounding headphones. The RP21 will provide a bit of variety. Besides, I love that vermillion color.