This is a different car but I figure we can apply the principles here too.

In my Saab has the 9 speaker Harman Kardon speaker setup, it actually is a pretty good system with the deck speakers being the only downfall.

The stock speaker system:
Speakers for AS3 / Prestige (four door) made by Harman Kardon
Location Size Impedance Amplifier power Frequency range
Instrument panel left/right 2.5" 2.5 ohm 2 x 16W midrange/treble
Instrument panel center 2.5" 5.9 ohm 16W
Front doors 6.5" 2.4 ohm 2 x 22W bass/midrange
Rear doors 2.5" 2.5 ohm 2 x 16W midrange/treble
Subwoofer 6" x 9" 2.0 ohm 2 x 22W sub-bass

I have replaced the front dash speakers and the clarity is awesome now. I upgraded to some Infinity Kappa 32.9cf and these are fine. In fact I may replace the rear doors with the same thing eventually. Is this a good idea...or will it be too bright?

The front door speakers act as a subwoofer, they are powered perfectly by the stock system and hit hard enough for me and have a very good range without any distortion. The rear deck speakers I can't say the same for, they have good midrange but distort the bass. I have bought some Infinity Kappa 692.9i speakers, I did an initial test and they seem have the same level of output but without the distortion (The ones I bought have to be returned as the one speaker is blown, they were used). I would like just a little more bass from the rear speakers since I now plan to get rid of the Infinity Basslink sub that has now died for the 2nd time. What are my options here? I was thinking about getting a small 2 channel 300-500 watt amplifier and push these rear speakers, but will that over power my other speakers? These have a -3db switch on them to cut the volume if you amplify them. Right now the system is about there, just the damn distortion the rear speakers have with the bass. There is a built-in amplifier in the car, will this play a role in adding this amplifier?

I should mention that I am replacing the headunit in a week or two with a Pioneer F700bt, most guys with a Saab say the Pioneer head units don't affect the sound except maybe even a clearer sound if anything.