The bass originally had an OBP-3 three band preamp with a mid select.
So my guess is
master vol pull passive,
pickup balance,
mid pull mid select ,
treb/Bass
From Nino:
VMJ4 #129
Weight: 8.70LBS
Specs:
Swamp Ash Body
Black Pearl Finish
Graphite Reinforced Maple Neck
Maple fretboard
Black Block Inlays
1 1/2" Nut Width
Aguilar AG 4J-70 pickups Aguilar OBP-3 preamp
HipShot Vintage Style Bridge
HipShot HB-1 Tuning Gears
Dunlop straplocks