you have 2 ways how to do it (actually much more, but these are the most obvious). 1) Classic Bass Control - see Timmy diagram. This method is simple, but this is not quite a classic bass adjustment, the more bass, the more fuzzy the sound, since this is a pre-distortion setting. 2) Active bass, after the amplification stage, see Way Huge green rhino diagram, you can just add a control next to the tone control, you can add it with a separate amplifier. This is a classic bass control and will work as you expect.