I've had this bass in for nasty buzz and hum that got even worse after someone else's attempt... Wires were very thin and brittle, shielding paint was somehow inefficient. So I went rewiring it all and properly shielding it. It is dead quiet now, and plays really well. I did change some of the stock caps (on the preamp and tone control) for better quality ones (these were like the plastic transparent ones found in many Boss pedals). I do like the active preamp, it's a nice feature on this good sounding passive bass. And the neck is really nice if you're into fast and thin ones.
I did trace the preamp and drawn another PCB for those willing to try it out or wanting to upgrade with other values/specs.
Here are the guts:
Here's my hand-drawn schem:
And PCB and layout (wire colors matches the one in the bass, diagram is available online):
(File amended and reposted below)
D1 and D2 are small signal diodes. I guess D3 is a protection diode.
Oh, and I did forget to write down the transistor type


Feel free to spot any mistake...