Got a similar problem. Sound is good but for some reason two thirds of the volume is coming through the left channel. I have compared ouput (from my sony nwhd5) with my sony cans (bought from amazon for about a tenner) and they are definitely defective. The sony cans are excellent and the sound well-balanced.
My sennheisser buds have not been damaged in any way and there is no intermittent/carackling sound which is usually the result of a wiring problem.
The fitting of these buds is not ideal but I was persevering but the sound to the left side is more annoying.
Did you get any joy from sennheisser? Did you return them to Amazon?