i had tons of feedback from my distortion pedals that i couldnt get rid of, i had an ISP decimator ages ago and i dont remember it getting rid of all of it, just lowering it, tried another one but its stopped working now.
went on tour with a mates band and tried their boss NS2, totally killed all feedback from their basses when they had it at the end of their chain, tried it with my stuff and it was still there...just quieter
then tried my gain pedals in the loop of the NS2 and SILENCE, i love it