The settings on a bass amp have a large effect on the final tone of the bass. Depending on the style of music and techniques being used, different settings can be better suited for some situations. There is no definitive best setup that will work under all conditions. A bassist can go pretty far with three basic bass settings without having to experiment.
A bass note is made up of a number of different frequencies. The equalizer (EQ) knobs adjust the amount that each frequency range contributes to the final tone. The Bass controls the low frequencies, which contribute much of the power behind the note. The Mid controls the middle frequencies, which contribute to the note clarity. The Treble controls the high frequencies, which affect the brightness of the tone. Too much or too little of any of these elements tends to lead to a poor tone.
The flat setup is the most basic one to use. For this setup, the Bass, Mid, and Treble are set to the same value. For this type of setup, 5 is usually a reasonable value to use. This setup can basically be used for any technique and style of music without any issues. It lacks the unpleasant tonal characteristics of more extreme settings. The downside to this setup is that it is not the most appealing tone, but it is functional.
High Mid Setup
The high Mid setup is a bit more appealing variation of the flat setup. The Bass and Treble are still left at about 5, but the Mid is increased to about 7. This gives the notes a bit more clarity and helps cut through the mix better than a flat setup. There is a slight risk of developing some of the bad characteristics of too much middle frequencies. The most common one is a honking quality to the tone. If this develops, it is usually a good idea to drop the Mid to 6-6.5 to try to eliminate it.
High Bass-Treble Setup
The high Bass-Treble setup is more oriented for slap-pop bass playing. For most other styles of bass playing, a flat or high Mid setup tends to be more appealing. In this case, the Mid is left around 5, while the Bass and Treble are both set to about 7. The relatively lower Mid value tends to lower the overall note definition, but the higher Bass and Treble give a bit more power and brightness to each note. This can be useful when playing slap-pop lines. This setup is best used for songs that almost exclusively use that technique, rather than for songs that have only a few small slap-pop parts.