I didn't get any clipping or distortion at 90% volume with my K401, but I just listened for about 2 minutes. It was also some choral music so not a lot of bass.
I have had bad clipping when playing some electronica from my iPod using a pair of old AT ortho headphones. Orthos are notoriously hard to drive but for most music the ATs are driven by an iPod at 75% volume and sound OK. When I played something with a heavy bass line, all of a sudden the music was almost unrecognizable. It took me a couple of minutes to realize what was going on and it didn't damage anything. I flipped through the whole album and it happened on every track. Tried a different album and everything was back to normal.
I guess the lessen is that a headphone that makes you crank the iPod for can cause the iPod to clip if the music suddenly needs more voltage swing than the iPod can deliver. I'm far from an expert in this though.