Why not add a parametric or graphic equalizer? Something like this Audiobahn unit should do nicely. There are cheaper units ($25!) that are available also. Like home audio, your amount of expenditure is directly related to your level of insanity. I've bought a few things from The Zeb and have been quite satisfied. Their prices are excellent.

I used to have a 900tc. Great car with the best seats of any car I've owned. What model Saab do you have?