Great thread for those who want to get a quick impression on a large variety of headphones.
Here's my contribution:
Full-sized (open and closed):
Sennheiser HD555 - Very comfortable and light, to the point where I always forget I have them on my head. The sound is very pleasant, warm, never fatiquing (due to slightly rolled of treble). Bass is not as strong as to achieve a really accurate and neutral presentation, but for the street price it's excellent. It cannot resolve bass detail as well as the M50 (see below) and lacks the solid bass impact that you get from speakers.
Sennheiser HD600 - More detailed treble than the HD555 but the rest sounds very similar. Heavier weight and tighter clamp on the head but still comfortable. Just like the HD555, its bass is missing the punch/impact of excellent speakers, and doesn't resolve bass detail all that well.
Sennheiser HD650 - A bit more bass than HD600 and slightly smoother treble (but not rolled off) but the rest is similar, including the missing bass punch/impact and detail.
Beyerdynamics DT770 / DT880 – Sounded similar to each other, with the DT880 being a bit more accurate. Both have that slight spike/shrillness in the treble that I don’t like. These are very comfy though.
Denon AH-D950 - Earcups not big enough so prolonged listening will hurt my ears as the drivers would press on my ears a little. The sound is signature Denon, with smoothly hyped bass and treble for a more dynamic sound, which definitely makes them fun to listen to, without being offensively inaccurate. Bass detail and impact not as good as the M50 (see below).
Pioneer SE-DJ5000 - Not comfortable at all (tight clamped earpads), with recessed mid's and a bit harsh in the treble range. Bass detail is a bit better than the Sennheiser HD models listed above, but that's about the only thing good about it.
Audio-Technica ATH-M50 - By far one of the best closed headphones I've ever heard--especially considering the price (no wonder reputable audio engineers around the world are raving about it). The sound is very neutral and accurate, with no boosted or scooped frequencies. Its ability to resolve bass detail is also superior to other more expensive open headphones, and it also can reproduce similar kind of solid bass impact that high-end reference studio monitors do--even the HD650 can't do that. The soundstage is smaller and the sonic signature is more analytical instead of lush like the mid/high-end Sennheiser HD series.
Equation RP21 - Very good for the price. Slightly 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.
Sony MDR-7509 / MDR-V600 – I have always disliked the typical Sony headphones due to the shrillness of the treble, and it’s still the same this time. Absolutely couldn’t stand it. Talk about bleeding ears and listening fatigue!
Sennheiser HD280 Pro - Way too overrated IMO. They tend to sound boxy. The bass is a bit anemic, and the overall sound is a bit limp and lifeless. At the same price, the RP21 sounds significantly better (although the HD280 doesn’t have that slight spike in the treble).
Sennheiser HD202 - for such a cheap price, it’s quite good. Bass is hyped, and lower-mids are recessed. Tiny ear-cups the just surround your ears are a bit weird–like someone’s hugging your ears.
M-Audio Q40 – Right behind the M50 in overall sound quality. 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 than others, but some might prefer the looser feel of the M50 and the RP21.
IEM:
Shure E4C - Smooth sound, but lacking bass, and I prefer to EQ the treble down a little for a less fatiguing prolonged listening.
Westone 3 - Worst purchased I've made considering the price. It's not horrible, but I'm all about neutral and accurate frequency response instead of heavily colored. The treble is rolled off too much for my taste (taking the kind of exciting sparkle away from music that needs it--like orchestral strings, brass blasts, jangly guitars...etc). The bass is hyped and unable to resolve any kind of detail whatsoever--just a muddy mess.
Shure SE530PTH - Pretty good balance overall, but the 2KHz~6KHz region can be very slightly fatiguing depending on the music played. The bass is nice and full, although I wish the sub-bass could be just a bit more prominent. Horrible cables--the worst ever. PTH device is also cheap and has bad connection/control.
JVC Marshmellows - For the price you shouldn't expect much--typical consumer grade sound with spiked treble/bass and fatiguing after a while.
Denon AHC351K - Not too bad for the price. The treble is a bit hyped but otherwise better than I expected.