Orbit -
Piezo pickups like the Baggs in your bass aren't sensitive to the kinds of hum sources like regular magnetic pickups, so it's very unlikely that the hum problems are due to your house electricity or the other kinds of things that emit hum (light dimmers, large motors, etc.).
If you plug in another instrument and don't have any unusual hum issues, then there's probably a wiring problem in your upright bass. If you haven't already changed the battery, that would be the first thing that you want to do. After that, you need to check all the connections to make sure that everything is solid. Beyond that, it would have to be a problem in the pickup or in the preamp/electronics module and would probably be difficult to diagnose with only that instrument (in these cases, you identify the problem by swapping parts with a known good set to isolate the faulty component).
Hopefully it will just be a dead battery or bad power connection. When the power supply is insufficient, you'll get a lower output level, which requires you to turn the gain up; when you increase the gain level of your amp, you'll increase the loudness of background hum as well.
Hope this helps,
David Fung
