What do these knobs do?

thefez

Bought my first bass off a friend. On three of the knobs there's a halfway point where there's a little click. What does that do? They are active pickups apparently. Looks like this if that's helpful:

mart

The three knobs might be active EQ, so the centre click is telling you the "neutral" position. But also one might be a pickup selector, with the centre point telling you the position where both pickups are on full.

Musicwolf

Bass, Mid and Treble controls.  Unlike a passive guitar where the tone just filters off the top end with an active instrument you can boost as well as cut frequency bands.

Funkfingers

Mart is correct at the second attempt.

Two pickup bass. Therefore, master volume (no centre detent), pickup balance, 2-band EQ. 

Additional EQ bands would require dual concentric stacked pots and control knobs.


EDIT: Chances are that the pickups are passive and only the EQ controls are truly active. 

NOTE: If the PP3 is flat, the controls do SFA.

ICBM

The knob at the front with no detent goes from Off to Loud. Normally best set to Loud.

The other one at the front goes from Fat/Punchy to Clear/Humming, by the look of it - set to taste, or to Fat/Punchy if hum is a problem. (It might not be, some of the narrow hummy pickups are actually also hum cancelling.)

One of the other two goes from Thin to Full-Sounding. Set to Full.

The last one goes from Dull to Hiss, with a useful detent in the middle so you don't accidentally turn it up beyond halfway.

Most bass players don't feel the need to touch them again after that.

Does that help?