Have you also tested the bass in different rooms (different "fuse sytems") and different locations?
I you want to keep the bass - canabis or MUCH BETTER a relaxing training might be useful! Thne buzzing remainds but you don't mind ...
The Mustang RI should have (good) S. Duncan pickups ...
Sh*t always happens - and I have heard of a Fender PU with mixed up cable colours! If your PU hums, maybe the cables have been mixed in Japan (but this is only 0.001% possibility!).
Tone knob works as it should.
A bass is supposed to produce sound - not silence!
Is the buzzing still to hear when a guitar player has put his Overdrive on?