I recently compared my Shure $150 SRHDJ head phones to my roomates $500 or so sennheiser hd 650s plugged into a $275 preamp and can honestly say the difference was hardly noticeable. I'm not an audio pro but I'm a musician with a good ear who has had a lot of kinds of headphones. For the money I can't justify sennheisers. Anything by Shure has been top quality, I've had a bunch of their stuff.
Get the HD 6xx or the HD 800 if you can. I'm not saying the 500 series are not good, but the 600 and the 800 are way up there.