My Bongo has the same issue as yours. The single coil hums but the bass is dead silent when I turn the blend knob towards the bridge pickup.
When I got my Bongo HS yesterday from EBMM factory for troubleshooting, the repair-tech wrote in the invoice that the hum is minimal and is normal for a Bongo H/S. He also A'B'd my bass with another Bongo just to make sure.
I'm not sure how loud the hum is in your bass but I experimented playing in different areas in my room. The hum is very apparent when I am in front of my amp and facing the cabinet but when I face the other way and move about 3 feet from my amp, the hum goes away. As Lord Bongo mentioned, the hum is due to environmental factors.
But if the bass still hums even when you move to different areas in your room, it's probably an issue with the silent circuit, but it could also be a problem on the preamp board that is preventing the silent circuit from functioning (as quoted from the repair-tech's reply on e-mail ).