First off, welcome to mp3car!
Now as for what you should do with your sound control, If you are just looking to remove the bass from the smaller speakers I would suggest a passive crossover, something like "BassBlockers" from crutchfield. Last I checked they made 3 different models with different cutoff points. If you really want to get into it more, you would be looking at getting an equalizer.
Another option is getting a computer that supports ASIO or is compatible with kxProject and use the carputer's soundcard to control all of the audio.