Hi Toshi,
I used this pre-amp in my JBA-4 build and it sounds great
http://www.ebay.com.au/itm/400368322849
You have room for 4 pots so they are master volume, balance, treble and bass. The circuit board box might be a bit deeper than the control cavity but you should be able to carefully chisel out a small channel. You will need a battery box so you may need help for that. I paired the pre-amp up with Tonerider jazz bass pickups and it sounds so bright and clear. At only $20 for the pre-amp will be cheaper to put the active system in rather than EMG pickups.