I was never a huge fan of the HD600, and when I bought one just over a year ago to replace the Alessandro MS Pros I had just sold, it was because the Sennheisers were much cheaper. I held on to it as stock for a short while but it was too uninvolving -- flabby bass, veiled treble, not very dynamic.
When I got the Cardas cable I was really astonished. I didn't think that it would change the sound so much. I guess I should have, considering I paid slightly more for the Cardas than for the HD600s (used). The treble and bass both became clearer and the bass became tighter.
Still, the headphones had too distant a presentation and not enough dynamics. Since solling them after buying the Grado HP-2, I haven't looked back. But if the HD580/600 has a sound you enjoy but would like to improve, you should really look into either the Cardas or Equinox.