I have a Warwick Corvette Std with active/passive J pickups, and recently it's been sounding quite buzzy below the 3rd fret or so on the E string, and also the B string.
This isn't fret buzz, but a buzzy sound coming through my amp, and it's worse when I play louder.
I know it's not the amp's problem because my other bass makes a really clear sounding low E through the amp, even when played loudly.
I have a feeling it may be to do with pick-up height or something like that, but I have no real idea. Also, the buzz seems to die off when I cut the highs, I'm not sure if this helps.