My bass is buzzing at low notes.

[Poop-Loops]

I noticed just now that my 5 string fretless (active pick ups) is buzzing at low frequencies. I plug in my dinky SX bass and it plays fine. So I detune to see if it's just really low ranges (it's a 4 string, passive pickups). Nothing. That rules out the amp. I plug my 5 string in again... voila! It plays fine. So I keep playing. Maybe it was a loose wire? I try to set my tone, and it happens again. Knobs? I plug my SX back in and play with the knobs. Nothing. I plug in my 5 string again. Clean. So I play with the knobs. There it goes!

What could it be? Bad soldering? Or something wrong with the pickups?

Oh, it effects my open E and B strings, the ranges in between. Higher strings like A are uneffected, so I thought it could be the amp, but it's not. =/

[Poop-Loops]

Well, it hasn't fixed on it's own (apparently time doesn't heal all wounds. ), so I could use some suggestions. I'm not afraid of electronics, but it would help if I knew where I should at least begin.

[dion mauer]

Buzzing? I assume you mean that the buzzing is coming from your amp and not the bass itself. The pickups are passive, but are the rest of the electronics passive as well? what about different pickup settings, bridge p/u soloed? neck p/u soloed? both p/us together? and while we're on p/u's, are they set to the correct height and are they mounted solidly (e.g. the p/u's dont move around, rattle, or interfere w/ string vibrations, physically or magnetically)?

have you opened the bass up to look for poor quality solder joints? it could be as simple as a loose connection somewhere in the wiring (and from what I gleaned from your post, this seems quite likely to me.)

another issue you may want to investigate, is the nut cut properly? can any of the strings move laterallly (side to side) within the nut slot?

another possibility, are you muting unplayed strings. are there any open strings ringing?

just some suggestions.