I bought a pair of cx300s a few weeks ago from Amazon market place for about £7, and to say that i was disapointed is an understatement - they were awful. i should of sent them back, but they did seem to improve a little, and with some cds they were almost acceptable, although the complete lack of bass could only be overcome by judicious use of the equalizer.
last night, i was doing some online research, when i came across the possibility that they were counterfeit. i compared all of the criteria, and they passed every one, suggesting that they were genuine, but they still sounded awful. Today, i bit the bullet & bought a new retail set. WOW!!! these headphones are AWESOME. With the equaliser flat, there is bass by the shedfull, yet the middle & treble are bright & clear. the new ones look identical, but audio wise, they are universes apart.
You may be tempted to buy cheap, and maybe there are genuine cx300s being sold cheap. Just don't say that you haven't been warned!