I have never even bothered connecting my K 601 to my iPod until reading your post, so it was actually quite interesting to hear them driven by a portable. The first thing I noticed is that I literally had to have the volume at 100% in order to get any life into them at all. Keep in mind I only had time to listen to a few tracks from one album, which isn't a particularly loud one (Sonic Youth - Daydream Nation). Even with newer, more compressed albums, I'm sure the output volume would have to be jacked up pretty high. The bass was almost non-existent, and the music heartless and dull.
Knowing what the same album is "supposed to" sound like with the K 601, all I can say is that an iPod won't bring these anywhere close to their potential. I personally can't imagine using full-sized headphones with an iPod anyway, and it would bug me to always have to use a 1/4-1/8" adapter. I would definitely seek out some easier-to-drive headphones, preferably something more portable and terminated with the appropriate connector, or some good IEMs instead of the K 601 in your case.