if you cant solve your problem, you might be able to correct the volume and tone with a BBE Sonic Stomp pedal.
http://www.bbesound.com/products/stomp-boxes/sonic-stomp.aspx
i have used it for guitar and it works great....adding volume and tone without coloring the sound. when you use the louder bass, click it off...when you use the quieter bass, click it on...
i assume they work as well for bass as they do guitar