Piezo pickups need to work into in a very high input impedence. 2 to 6 meg ohms if my memory serves me right. Most guitar amps have a much lower input impedence and this is why the sound gets harsh and tinny. Guitars such as Takamine with under saddle pickups have a pre amp built in which takes care of this. If you are into diy electronics you can find circuits that will work off 9 volts and fit into small boxes. Some effect pedals will do this as well.
I once got fair results by putting a 2 meg resitor in front of a simple high gain pre amp. What does the maker recommend?