Pickups convert string vibration into sound. The distance between pickup and string will factor into the overall sound of the bass. Preparation of the instrument is where to start.
Make adjustments to the truss rod (neck straightness), action (string height along fret board) and intonation. String height over the pickups is part of the process.
The general rule for the space between strings and bass pickups is between two and three millimeters---about 1/10 of an inch--or not quite 1/2 of a PEZ candy. Further adjustments can be made from here.
Once adjusted, plug the instrument into a bass amp. Maximize any amp controls, as well as those on the bass. Try everything, with an ear to finding a balanced output.
If the strings are too high, tonal response may be weak. If too low, sustain suffers, for the pickup magnets pull the strings, adding an almost warbled sound. Each player has his or her own tastes, but chances are you may find that the tone you always sought was already in your hands.