i've had the px100. and now i have the koss ksc35, which i think have the same driver as the portapro (dont know about portapro2). both are open. while the px100 is more durable, the ksc35 have much better sound overall. both of them are bass driven but i think i prefer the bass of the ksc35. the px100 lows are kind of dark. the ksc35 also have a more refine treble. in other word, i am amaze by the ksc35 suond quality. it is as comfortable as the px100, but not ideal if you wear glasses. also it feels fragile.
edit: oh, i think i was supposed to be talking about the px100. first, its very durable and portable. its more comfrotable than the px200 and i prefer it sound over the px200 too. it have a warm, slightly dark sound. it have a good amount of bass, more treble power than the px200. maybe taking away the slight boominess of the bass could yield better mids.
some people actually prefer it over the portapro because they said its more balances. in my opinion, since its portable and cheap, the different is so little so it dont matter and the koss have an overall richer sound to my ears.