My understanding is that the use of the name Bose in a stereo system entails varying degrees of involvement, from simply engineering, to providing actual components. I've read that in the A6, they provide the speakers and amps, and that Panasonic manufactures the head unit. I'm not sure what has changed for '02, but the head unit is obviously different, and I've read negative comments about in comparison.
I have no desire for the audio in my auto to be perfect, but only that it allow me to pleasurably listen to the jazz artists I enjoy. I too noticed that the fidelity seemed to improve as the volume increased. Since I don't like listening to excessively loud music that wasn't helpful. The sound system seemed to rob most soft vocals of their resonance. An ideal example would be Sarah Vaughn's recordings. If anyone is familiar with this vocalist, she had a marvelous instrument. Her voice was resonant, full of harmonics and she had a 3-octave range. The problem seemed to be that the system magnified the louder notes, and curtailed the softer ones, thereby robbing her voice of resonance and making it sound flat. This Bose system was marketed as having a very "Clean" sound. Perhaps this effect was intended to get a more abrupt cessation of sound to achieve that.
Pre-model year '02 cars with the Symphony Sound System have some hidden features. I don't believe the new sound system does. One of these features is an 8-band graphic equalizer. The default setting for each band is 5. I accessed this feature and set it up as "67766776" This has substantially improved the fidelity of the type of recordings to which I listen. The vocals now sound much better with more of the fullness and richness that I obtain from other systems. I've had the chance to listen to Stacey Kent, a favorite contemporary singer, live. Using that live performance as a baseline, I'd say that my adjusted system is about 90% satisfactory.
If you have a pre-'02 Symphony system, you may want to experiment. This is a site in which someone kindly provided instructions for the hidden features: