The whole city is wired like that, I don't like it either. When I started playing all amps and most outlets were like that but they also all had polarity switches which is the same as switching the plug. The way of least hum was the correct polarity switch position, three prong plugs with an added ground didn't become common until the mid 70's.
The T-Bird is the only bass I have here that hums like that, I will check diming the bridge with the neck down but it doesn't pick up any noise at all other than 60 cycle hum which is not usually what you hear, you usually hear 120 cycle hum and a lot louder. The neck sounds nice and fat and picks up no extraneous noise, those two things tell me the pickup is a wired correctly.
OK I just reversed the plug polarity on my amp at home, turning either pickup to full stops the hum, it only hums with both volumes below full. It's something internal as at the club there is no buzz or external noise with any of the basses.