I listened to the PK1s and PK2s at HeadFest in SJ being driven by a current (at the time) Head-Direct amp and my iPod. I haven't heard the PK1s since then.
I bought the PK2s. They have a nice midrange and are a little rolled off at the extremes. What they have at the extremes is tight. They have a decent soundstage but not the presence of full sized headphones (, which don't have the presence of speakers, which don't have the presence of live performance). An iPod drives them well and I find I listen a lot to them due to the convenience and their musicality.
What I remember about the PK1s is that the extremes are more present. I use a piece of organ music from the Burmester CD II to see how phones do with complex pieces of music. The PK1s sounds muddled to me in this test while the PK2s were much better. This and the fact that I didn't want to be tied to an amplifier are what led me to buy the PK2s.
Hopefully some PK1 owners will give their impressions of them. There are threads on HF about them too.