Use a magnetic pickup! Sorry, but it's the easiest route...piezos without a pre-amp an be very unpredictable.
if you can still remove the piezo.. take it off .. lay down a bed of hot glue .. then the piezo on top .. then another layer of hot glue . that will numb the feedback .
if the disk is glued on . just try caking the top with hot glue . or put another new one in and hot gule it .
or swap it out for a mag pup .
ps .. piezo disks dont like too much distortion .. changing your amp settings may help too .
I use the hot glue gun with good success. Mine will hum if I am close to the amp, but otherwise sound good.
one of my amps. a Marshall AS50D, has a feedback defeat/limiter switch to use with acoustic guitar piezos, it is adjustable and has worked with rod and round piezos very well - i'm not saying buy the amp, but the unit must be available somewhere for sale seperately...i've also seen somewhere on this site many months ago about a home made feedback limiter control box by someone for use on stage, maybe Shane Speal ? (-:
Wire 2 in parallel, then embed them in the bridge above the lid and outside the box. This has worked very well for me.
Yeah, what he said.....works very well for me