Ok, I have received the CX300B's and have spent a few days with them. Build quality is decent, not much to them really. The package is quite bare, not even a soft carry case is included, headphones and some different sized tips and thats it. I've tried them with 2 sources so far:
Motorola SLVR L7 with 512mb transflash card. MP3 files at 192k CBR. Good sound, decent bass, slightly rolled highs. Mids seem a tad recessed. Not bad overall, better than I remember the EX71's.
Apple ipod Shuffle 512mb. MP3 files at 320k CBR. Again, good sound. Outstanding bass, this combo is like having a subwoofer strapped to your head! Good highs, mids seem decent.
Overall I'm happy with my choice. I definitely wasn't expecting this kind of bass from a semi-canalphone. Isolation is almost what I wanted, these isolate just slightly more than the EX71's do. Recommended for bass heads on a budget!