I have a Z3 roadster with the HK, and I have the same problem. I went to the stereo shop where I purchased my home theatre system to see if they could remedy the problem. They told me the only thing that would work was to reduce the bass and volume. They also said that they didn't carry that size speaker, that it would have to be replaced by the dealer. He did say it didn't appear to be a blown speaker. Anyone else have any ideas on how to fix this with some aftermarket speakers? When the top is down, I have to turn the volume up quite a bit to hear the radio, and the distortion is horrible.