I used the feedback killer in the Behringer Ultracurve once in a live situation and it did something weird, when the first freq started to ring it caught it after a couple seconds like it was supposed to and applied a 3 or 4 db cut, cool, but when I went up with the gain again and the same frequency started ringing it didn't cut it any farther, just left the same 3 db cut. I ended up not using it and just ringing out the monitors by ear.
The problem with the feedback exterminators is that when you use them with good monitors that have a pretty flat response you have to crank the gain pretty high to get all 6 (is it 6 or 4 ?I cant remember) filters locked on their frequencies this can sometimes cause you to blow drivers. If you dont lock the frequencies it will hunt and kill random things like keyboard pads etc.