| Yes, it's an impedance adjuster, and yes, you would set it "by ear". Note that AFAICT it is not designed to alter the output impedance of the bass (though it probably does, to some extent), but rather to alter the input impedance that the pickups "see". IOW it's good for adjusting the tone/character of the pickups, but not necessarily useful for making a fuzz more compatible with an active bass. | 
