Over the last few decades I have played with piezo pickups with a certain amount of success. There are two important points required for their successful use. First these pickups work best when under pressure. This is why acoustic guitars have their pickups under the saddle. The strings press the saddle down on to the pickup and onto the guitar front. The increase of pressure gives a big increase in signal.

Second is the output from piezo pickups does not match the input of mixers or guitar amps. I'm probably going to get these figures wrong but the basic idea is right. Microphones work into 200, or is it 500 ohms resistance? Guitars work into 50, or is it 500 kilo ohms resistance. The point is that piezos work best into 2 to 5 plus meg ohms resistance. Put them into less and the sound is shrill and weak.

Another opportunity for me to be shot down in flames here, but one solution is to put a 2 to 5 meg ohms resistor in line with the output and then pre-amplify that so it gets heard through the amp, but the better solution is to get an impedance transformer circuit to match the piezo to a mixer or guitar amp. These circuits are quite simple, an 8 contact IC and 3 or 4 resistors and capacitors. They can be found online are within the capabilities of the hobbyist. I've done loads of these in the past buyin g the stuff from Maplins UK. I've even made my own pickups. I've probably got the old circuits filed somewhere if you would like to try them.