I own a 1999 A6 quattro. I just recently changed the battery. Since I'm not the original owner (or even the 2nd owner) I do not have the security code to reactivate the sound system once the battery has been disconnected. To get around this, I kept current going to the battery cables with a charger while I switched out the battery. Everything went fine as far as not getting locked out of the stereo. I have now noticed that the system sounds different. It is very bass heavy (even with the bass adjustment turned all the way down). I still have mid & highs, and all speakers are working with no distortion, but I can't bring the bass level down enough for a proper frequency balance. Is there some way to calibrate the system's bass adjustment? Or is there an input sensitivity adjustment on the sub-woofer amp? I don't understand what could've happened.
additional question: I was told there is some way to make the display show a spectrum anylizer, but it's not mentioned in the manual. Has anyone out there heard of this? Is it only on certain year models?

 

This may help! http://www.ibiblio.org/tkan/audi/radios/symphony.html (down at the Hidden Equalizer part). If this isn't the problem then i've got NO clue. May be an actual problem with the stereo.

 

lol...

 

I was able to access everything, but the system didn't respond to the hidden EQ, SVC, & VOL. It also no longer recognizes the CD changer and won't play it. When I play a cassette there is an oscillating static that can be heard in between songs, sometimes during a song.