Grado SR-225 or Sennheiser HD580?

HD600's over HD580's for sure. The 600's have crisper resolution, better frequency extension, and more natural, better integrated sound overall.

The SR series are a little darker with way less rez than the RS series. If you are serious about headphone listening, I would suggest you have a listen to the RS 2. They are about the same list price as the Sen 600's, but IMO, have better rez, bass and truer to timbre.

However, if I was to choose between 225 or 580's it would be the 580's. But again, I would spend more and get the RS 2's.

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.