The mid-range control is centered at about 500hz. The design of the bass and treble controls are different, so they don't really have a center frequency. All the controls are interactive, so the freq's cut and boosted depend on where the others are set.
That is about as technical as I get. I usually just turn a knob until it sounds good.
Thanks Greg. Interesting concept, it sounds advanced to me but it probably explains why I don't miss the sweepable midrange I've had on previous basses. The preamp certainly is very muscial. Do the freq's also depend on the coil switch?