I have a 2006 CLK350. IMHO, the audio quality is mediocre! There's not enough 'top end'...sound is muffled and muddy. Increasing the "treble" just makes it worse...harsh and shrill. Has anyone upgraded this (Harmon-Kardon) stereo system? I've been told that the speakers are optically conntected, with makes it almost impossible to simply replace speakers. (I suspect each speaker has it's own integrated amplifier, though I'm not sure.) Are there any options for improving the sound system?