Does the bass have an active bypass circuit (active/passive mode) or is it 100% active? The reason I ask is, it might be your preamp has gone bad.
It’s also possible that you have a bad wire (a break under the insulation) somewhere probably going to your output jack. I’d recommend leaving the back cover off and gently wiggling wires individually while you play to see if it changes anything with the volume and isolate it to a bad wire.
Do your pickups pass the tap test (with the bass plugged in tap on the pole pieces with a screwdriver and listen for noise through the amp)? Is it a volume/blend setup or are there separate volumes for each pickup? If it’s volume/blend, it could be a problem with either of those pots.