How to adjust vol vol tone controls?

[Spiketronic]

Trade it for a Precision. One knob will controll your world. Easy-Peasy.

I’m a jazz bass guy all the way

 

[wiscoaster]

I agree with the pedal solution. That seems perfectly suited to your need. Adjusting one or the other of two volume controls on a two-pickup passive bass affects the overall tone more than just the overall volume. If you want to adjust only the volume in one quick adjustment you need something outside the bass that affects only its output. So ..... since you don't want a pedal, maybe somebody could build you a little pass-through volume control box with a single volume pot - the control box plugs into your bass's output jack, and you plug your cable (or wireless) into the box's output jack. Or it could just be hardwired to the guitar end of your cable. Just an idea for an alternate solution that worked for me in a different application context some years ago.