Are you using that same Rig with any other basses? If you are then I assume that you only have that problem with this bass. Another thing to ask, is are you plugged into the same electrical outlet every time? I only say this because I like to ensure that the grounding of the outlets is satisfactory for basic safety considerations, etc...
Regarding sources of hum and interference, one time I found that turning off certain lights and other things in the house reduced the noise. In other words, you may want to consider looking at the basics of the grounding and interference where you live.
That said, a bass with grounding and shielding issues will be prone to those problems whereever you take it. There are a lot of great threads in Talk Bass on how to resolve these problems. You should run searches on "grounding" and "shielding" to find more answers on how to approach these projects.
