Originally Posted by JimmyM
Really, what does it matter how hot the output is if the bass sounds good? Just turn it up. Don't associate good tone with hot output. They're not related.
JimmyM is 100% correct with his statement.
The Fender Jazz Bass is not a high output bass guitar, compared to the P Bass.
The reason why? The neck and bridge pickup are wired in parallel. So, if the neck pickup is 7k ohm and the bridge is 7k ohm, when the volume for both is full, the combined ohms for the 2 pickups is 3.5 k ohm.
Compare that to the P pickup at about 10.5 k ohm.
That's why the Jazz Bass has less output. Turn the volume or gain up! Problem solved.
(if you turn off one pickup or the other and play only one pickup, you'll get the power of 7 k and you'll notice that your J bass is slightly louder than when both pickups are used together.)