Grado SR-225 or Sennheiser HD580?

Depends on your taste of music my friend. If I were you I'd get the HD 600 since they're really a more "rounded" headphones meaning they can be for listening everything.

Be warned though! These headphones really shouldn't be listened unless you have at least a couple hundred dollar headphone amp + preferably new cables to replace the stock ones such as Clou cables, maybe the new cardas?

Note: http://home.t-online.de/home/meier-audio/
Here's a good place to buy HD600s as well, also see www.head-fi.com for more info on headphones in general. ;)

Sennheiser has the better quality in sound.

I'm listening to Grado SR-325's right now with the Grado RA-1 amp. Too amazing. The Senn 600's are also excellent (also driven by the RA-1 but with the knob cranked), but the Grado's seem to put me right into the music. If you don't mind things in your ears and can be isolated from your enviromment, Etymotic ER-4s canalphones are totally accurate and sound wonderful.