Add ACROSS - to get a little more Bass. Add In-Line to add more Treble - with consideration
To Add Bass - Start small - it'll "lack Treble" and sound muddy if you use too high of a value.
To add more Treble - do the opposite - the smaller the In-Line cap is, the higher the tone it passes will be - but you need a rather large uF (Microfarad) value to start out to find the right "Bass Cut" anything below 0.047 (473 Disc) may not allow enough audio to pass and be weak sounding
Usually on those "Coffin types" - 560pF (561 Disc) to as high as 0.0022uF (222 Disc) ACROSS to kill (Roll-Off) the Treble.
But depending on what your radio is, you might get a little more Bass if you use those Goldfinger mods.