I reckon it's the body of your bass picking up the vocals and then feeding back through your own bass amp. Sweep your notch filter frequency until (hopefully) the feedback stops.
Also try your variable high pass filter (on the Fishman, they call it 'low cut'). N.B. the knob controls the cut-off frequency, not the amount of cut. Turn the knob up until (hopefully) the feedback stops - even up to the max if needed. I sometimes HPF at 160Hz, which is the Fishman's max frequency. If your instrument now doesn't sound as full, you can still increase the 'bass' knob.
Some people have had results by blocking their f-holes with foam. That might be worth a try, although for me this didn't make a difference.
Is your bass positioned right in front of your amp? Can you try moving things about so that you're further from your amp, or at least slightly to the side of it; or elevate your amp considerably so that it's firing over the top of the body of your bass rather than straight into it?
As well as your amp, is your bass also going through the PA? If not, is that something you can consider doing - this might allow you have a lower stage volume on your amp.
A larger change to consider is the setup of your bass and playing style/technique. If you were to play unplugged, are you getting much acoustic volume? Or are you tickling the strings and relying on the amp to be heard? The higher the volume is on your amp, the more chance of feedback. It's going to vary for each player/instrument, but for me, a slightly higher string height and more robust (but not heavy-handed) pizz technique resulted in better tone and more volume, which meant lower volume needed on the amp.