I have heard people say that 500k pots have more "throw" to them. I believe they are implying a broader dynamic range. If you really want to cut more treble and increase thump, try a higher value for your tone capacitor. Most stock basses have a 0.047 mf capacitor a 0.1 mf cap will cut more treble and provide more of a bass boost. This is a very inexpensive mod that may get you where you want to go.
Also, if you do opt to change out your pots you will probably want to use a linear taper for the tone control and an audio (logarithmic) taper for the volume control.