BiffsBizNot sure. I got the PK2 here, and they fit my outer ears perfectly, as do the Edifier P180 earbuds and Apple earbuds. The PK2 treble is a little choppy, but the bass is very, very light. I have a 10 db boost at 65 hz on my equalizer, but that basically adds warmth, not any significant impact. And they fit perfectly in my ears. Maybe for people with small ears (mine are average), the buds sit very close to or against the ear canal, and maybe that helps boost the bass. I suppose the PK1 could be better than the PK2, but I'd expect it to have a light bass compared to a comparably-priced but fairly neutral headphone, like the AKG K553 or Philips X2.
dalethornI've been using the Edifier P180 recently as well, and they have replaced my Yuin PK3 and PK2 mostly because it has a mic, and the sound is almost comparable.