I have a Grado SR125i. It cost 165 AUD (plus 43 AUD for a 4.5 m extension cord). I really like them although I don't have anything to compare them to as they're my first serious headphones. I feel the whole frequency range is equally represented and no details are lost. After using them for a while, my ears hurt a bit but with a couple minutes of rest, I'm good to wear them again for another album. On the other hand, my girlfriend can wear them for any length of time, so this seems person-specific. I should also add I only use them at home. They're not very nice looking I guess (although I have no problem with their design) if that's a concern for street use.
I know Grados are considered very good quality for a low price.
As for headphone amps, I don't have any experience with them. From what I've read, even headphones that are not power hungry often benefit from one because the headphone output on a lot of amps is not the same quality as the speaker outputs (many amp manufacturers consider them less important). I was told by someone I trust on these matters that the Grado+NAD amp (what I've got) combination is a great match though, and I've also read NAD amps (and, if memory serves me right, Cambridge Audio amps as well) actually have good quality headphone outputs.
