Altering tone...
The previous response is correct in that this circuit is tweaked so as to as transparent as possible. There are exceptions in such cases as the active electronics in a regulare electric (NOT ACOUSTIC) guitar such as the BC Rich series, which intentionaly color the mids, but this is not the case in a bass guitar as this would result in a very muddy sound on a bass guitar.
The tone you acheive from your instrument is a rather complicated combination of the qualities of the material the instrument is made from (woods, metal used for the bridge, etc) and the pickups. There are other variables but these two determine your tone more than anything else. Since you obviously can't change the materials your instrument is made from, the pickups would be your next logical step in altering the tone.
If the active electronics have some form of Equalization you may be able to also change the components that set the boost / cut frequencies of the EQ, but this is not always possible since some electronics use modules that are embedded in a potting compound that makes access to the components impossible.
At any rate, I would be careful what I changed as the active electronics used in these instruments is rather delicate and easy to blow out. FET input opamps are sensitive to static electricity!
I hope this has been helpful,
Experiment, Explore, Keep the music alive!
Mad Soundman
Sound System consulting, design, and installations