Lots of people have reported that their BOSE 6x9'* sounded fine new, but quickly lost their ability to produce good bass. This apparently is the result of a problem with the speakers not being able to handle the sustained output of the BOSE amp (not sure why). They don't technically fail...they just don't sound great. This is where you get into a dispute with Bose, which feels if the speakers haven't failed, then there isn't anything they can do.
I'm pretty sure that Bose has heard about this issue from a lot of GM owners.
Y2K Midnight Blue SSEi
White Cathodes in Grille/ Blue Cathodes Under Seats