Here's how I fixed my MIM Fender Jazz Bass single coil hum. Jazz basses (and single coil guitars) should be hum cancelling when 2 pickups are at the same volume. My bass had a lot of hum even with both single coils at the same level.

It turns out both pickups had the same magnetic polarity. Flipping the magnet of the one pickup turned out to be an easy fix, and got rid of my bass guitar's hum!