FYII have used my K601 out of my girlfriends iPod Nano. It drives them suprisingly well with just as much bass as my Gilmore Lite. Obviously the sound is not nearly as detailed and refined, but it is actually quite smooth sounding (the the iPod gently rolls off the top and maybe a little of the bottom). It more than drove them well with the volume bar at 50%. Given that, obviously they sound better with an amp (sound great with both my Gilmore Lite and portable Rockhopper PIMETA), but you can get away without one if necessary. The only thing that had trouble driving them was an old Sony PCDP I have. Seems you had to have the rolling volume at ~8 (10 is "MAX"), whereas with my K81DJ's or HF-1 I only had it at ~4.
The K601 is much smoother than a Grado. I ended up selling my HF-1 because I came to the conclusion that the K601 was a superior can for both my music preferences and for all music in general. Top that off with the fact that the K601 is extremely comfy and well you got a great can.
Listen to the music and enjoy. The ultimate mod for the K601 is to listen to them in a nearly silent room (late at night, computer off, windows shut, fans off). Dim the lights and close you eyes. You will be blown away at how REAL the music sounds. Oh I forgot to mention the K601's have a huge soundstage. They sound very BIG. You'll love em!