I have a 4003S bass and I love it, but I've been noticing a slight crackling (that's the best I can think of to describe it) noise when I turn the volume knobs.
It seems to be getting worse, and seems to only happen when a note is ringing out. If the bass is quiet and I turn them the noise isn't there, but when I play a note and then turn the volume knobs it is there. The volume for the treble pickup also sometimes drops dramatically without even turning the knob, just touching it, and with some slight tweaking then comes back.
Anyone know what might be wrong? Anyone had these problems too?

The first thing I would try is cleaning the pots with a pot and switch cleaning product like Deoxit. If that doesn't work, which it should, you may need a new pot.

To clarify, Deoxit is a "cleaner/lubricant" - the ONLY type of treatment that should be used on pots. "contact cleaner" will destroy them by removing the lubricating oils that protect the contact elements.

But this type of problem is more often due to a poor solder contact at one of the ground points or even a bad cable, input jack soldering (or mounting) at the amp or poor V1 preamp tube contacts.