Grado SR-225 or Sennheiser HD580?

YES!...in answer to your purchase of the Senn. HD-600. Happy Tunes!

I have both models, and although the Grados sound great, the Sennheisers are much more comfortable. They envelope the ear, whereas the Grados clamp down on the ear. It makes me feel like my head is in a vise. Also the Grado foam material earpads degrades after a couple or 3 years and has to be replaced. The HD580 has a very flimsy but long long cord. The Grado has a crystal clear midrange and somewhat elevated bass, while the HD580 is a little more recessed in the midrange, and still has good bass. To me, I listen 10X as much to the Sennheisers than Grados simply because of comfort.

I've heard that you can improve your 225 by getting the much better pads they use on their entry level SR-60s. Supposedly about $10.

Sennheiser -- you won't be sorry.

The pads from the SR-60s deffinately help the Grados. I use them on RS-2s. -- The HD600s are worth the difference over the HD580s IMO. The lack of resonance from the carbon makes a difference. The also have considerably more bass. You really have to want to listen to Grados to put up with the discomfort level. The Sennheiser is a good choice. You can check out frequency response curves at the Headroom website -- http://www.headphone.com/layout.php?topicID=10