Music Man - taming the highs with a capacitor?

I don't have an answer for how to modify the preamp's treble frequency, but there's a passive wiring trick that will lower the resonant frequency of the pickup, which could help mellow out the overall sound.

Simply wire a capacitor between the pickup hot wire and ground before the pickup reaches the preamp. (If you place this loading capacitor after any of the active components, it won't work.) Try .001uF for starters; to lower the resonant frequency further, go up to around .002uF.

 

Maybe this thread will help?
DIY Stingray 3EQ Preamp

 

With tiny caps like that, it's not about bleeding treble to ground in the sense of a traditional treble-cut cap; it's about loading the pickup to shift the frequency pickup's natural resonant peak. (Pickups form an LCR circuit; adding a bit of capacitance changes the circuit and therefore the sound.)

Oftentimes, when people go shopping for aftermarket pickups, what they're sometimes buying---without knowing it--is a lower resonant peak. So "Brand X sounds warmer, has better mids, etc.," can be a reaction to a different resonant peak.

And as the linked post above mentions, the resonant peak can be smoothed or flattened with resistor, either on its own or in series with the cap. (A small resistor, around 6 or 8K Ohms will lower the peak without flattening it entirely.)

As to where to put it, if the treble pot is what the pickup hot wire reaches first, then yes, that's a good spot, between the hot input lug on the pot and ground.

 

Why not just rewire the pickup in Series?