The Yuin PK1s are very special just like its new sibling the OK1s. The PK1s need plenty of amplification before they begin to sound their best. Once you pair them with a good amp you are in for quite a treat. The Yuin PK1s have a more fun sound signature than both the OK1s and OK2s because of increased bass and more slam. Overall the PK1's main problem is that they really need a good performing amp to sound great. I think the PK1s will be able to satisfy most people when it comes to amount of bass.
Their sound quality is really good and they are not as pricey as its newer rival the OK1s. If you are looking to get a sweet earbud based portable rig the PK1s are definitely worthy of your consideration. And if you are a basshead you should go for the PK1s rather than the OK1s.
To me the PK1s can definitely hold their own compared to the OK1s because of their relative low price. The PK1s are way more bass heavy than the OK1s at the cost of some high range definition and bass definition.