I have a factory 1999 456 MGTA with no mods. Recently, the car audio gets distorted with very little bass response and a noticeable crackling sound. Low volume levels don't seem to present a problem but increase the volume and the sound gets distorted. I thought it could have been a blown speaker but I don't think all the speakers would blow simultaneously. Does the 456 have a subwoofer? Any suggestions on what the problem might be?