You haven't really described what your initial problem was, so I'll throw this into the mix.
I've heard many stories from reputable (?) folks about the output impedance of piezos being too high to work well with passive controls and the need to buffer/pre-amp/blah-blah-blah/bullshit/etc.
Now, I'm not 'totally' disagreeing with those people because, quite possibly, they may be right. However, I have lost count of the times that I've heard people (myself included) get amazing results without buffering. Don't believe me? Give Dave Wendler a call, he's probably been doing this even longer than I have.
Anyhow, despite what I say here, ''your mileage may almost certainly vary'' with passive tone controls on piezo transducers.