You need to balance the volumes of your pickups.The heighth adjustment screws serve this purpose. Getting them to relatively the same volume will allow you to have just one volume setting on your amp and contol all the tonal changes from the bass.You CAN get them perfect if so desired(I liked mine as a blend,so one might be a bit louder for effect)...you do this with a simple voltage meter set on the DC setting at a very very low scale.One with a needle works better than a digital.(thats only MY opinion).Other than that,simply plug in at a low volume and adjust them up and down till they match.

After you adjust them, if there is still a drastic difference, you may have either a grounded or cut coil wire which is still passing some but maybe not all of the coil going out to the amp. Either that or there is something going on with the tone or volume wiring which is causing a problem. You may need to take it in for repair if this is the case.

MistaG is right. It could have a bad volume or tone pot. It could be a poorly wound pickup or the magnets may just be weak.

Bartolini and Bill Lawrence both make excellent replacement pickups for Fenders guitars and basses and they're affordable.

Also keep in mind this is a Standard Jazz and the quality of it's components are typically less than those shipped with the amercian made models.

But again, you might want to have your local guitar shop measure the tone and volume pots for you to see if they need to be replaced. That's a very affordable thing to do as well.