I have the PK1 and PK2.. They are both great, the PK2 doesn't need an amp, the PK1 sounds great without, but really benifits from one.. The Pk1 has a better structured bass and is very well balanced.. So is the PK2, but slightly less detail and lighter in bass..
This is how I hear it anyway
The pk3 is supposed to be great as well and rather cheap but I haven't heard myself..