That's a very good question. I have both, and A/B'd them when the PK3s arrived the other day (without any burn-in). Is the PK1 worth $100 more? I have to say, it's a very tough call. The PKK1s have a more "refined" sound, I believe (I know, not very definitive), but I was in awe of the PK3 sound amped. I started to wonder myself, in fact, if the PK1 sound sig was worth $100 more. But hey, I have them both now, and I do prefer the Y-cord of the PK1s. I will use the PK1s more, but when I don't want to use an amp (which is becoming less and less these days), I'll use the PK3s. For $25 via the group buy, the PK3s are about the best value in portable phones, in my view.
Let's put it this way. If budget is an issue, just amp the PK3s and you no doubt will be happy. If $100 isn't a big deal to you, then try the PK1s.