That's what he started out doing.
My point with that, since that's how I've primarily used my Jazz Basses since 1966, is that the problem is probably NOT with his bass control knobs, but with his EQ settings on his amp. Presuming he doesn't have some strange aftermarket pickups on his bass, it's not the pickups causing his problem.
Plus, when you use the pickups to set your sound instead of your amp, you really limit your ability to make changes on-the-go with your bass. If you run them both of them open all the way and the tone all the way up (remember these are passive controls that only cut - you cannot boost anything), you have lots of choices for changing your sound without going back to the amp in the middle of the song.
Also, if he hasn't already figured it out, hand position on your plucking hand makes a big difference without even touching the control knobs. Pluck the strings back by the bridge pickup for a more treble sound. Too trebly, move your hand forward a bit. Need a smooth mellow sound, play up to or even above the neck joint. Too mellow? - move your hand back towards the neck pickup a bit. Too mellow of a sound still, move it farther back.
Can't make the adjustment you need by changing hand position, then try rolling off some of the volume on the neck pickup and try again. And just because he turned it all the way off and didn't like it doesn't mean it won't sound good with the knobs set somewhere in between. These control knobs are not just on or off, there are a lot of positions in between the two extremes.