I've been a Cx-300 diehard for the past 10 years..
I've gone through 3 sets of Cx-300.. because I found that they're very neutral, by the books german, with just enough bass..
Finally my last pair of cx-300 got rolled over under my computer chair, So Now I got the Cx-300-II
Build quality: Good, good'ol sennheiser quality
Sound quality: Good, same sennheiser frequ-response/ signature. Treble response is louder