I second the ATH-M50. I bought these 2 years ago to use as my mixing/monitoring headphones at work, but they have quickly become my hi-fi listening 'phones as well. IMHO, they are the best of both worlds.
The construction is great. Very sturdy plastic, and a strong metal band over the head. They hold up very well in harsh environments and during extended periods of high-volume use. And despite 2 years of this abuse, I haven't even had to replace the ear pads.
They are comfortable! And I mean really comfortable. I often spend hours at a time with these headphones hanging off my head in various positions. They can be just a bit tight, but bending the band a bit fixes that issue. The other thing that is great, is that even though the pads are vinyl leather (I think), my ears don't get even the slightest bit sweaty.
Anyway... enough with that. On to the sound quality. These 'phones are incredibly consistent at any volume level. From quiet listening at home, to listening to a mix at a loud concert, I can hear every detail, and the frequency response barely changes at all with volume. (It looses a bit of bass at very loud levels) There are many songs where I think the sound of these headphones is even better than my rather expensive speakers.
And the bass. Wow. The technical sheet specs the low end cutoff at 15Hz, and I believe it. This can handle the anything from the smooth sound of an upright base, to the bassiest dubstep you could find. I have actually been able to feel the bass vibrating part way down my neck, albeit at absurdly loud volumes.
The highs are just as good, if not better. The 28kHz top range is crystal clear, and where I've heard other headphones, even speakers, become harsh, these phones consistently stay clean and distortion-free. I don't know what else to say, but they're ********* awesome.
Well, that turned out a bit longer than I was expecting, but as you can tell, I really like these 'phones. I have looked at many other headphones, and I think that the M50 is by far the best pair of headphones that can be bought for $150 (ok, 160).
edit: Oh, yeah. And they don't need an amp. Anything I plug these into has more than enough power to run them. But as usual, a nice headphone amp almost always helps make any headphone sound just a little bit better.
Edited by vockleya - 10/4/12 at 11:02pm
... Too bad, because they're excellent IEMs though may just be a little beyond your initial budget. I've never checked out the Cosmic Ears Flex though, and reading reviews they seem to be really interesting (custom in-ears at this pricepoint!). 
