Check to see if this is caused by back-EMF from the woofer. Certainly, the solution is to refrain from clipping the amp. But I'd like to know if, when the woofer is decoupled from the circuit, the piezo "buzz" goes away. I've heard 'em "chirp" from back-EMF in certain implementations. So try either bi-amplification to decouple the two systems, or simply temporarily removing the woofer from the system for a "chirp test."This situation was covered in some detail in the post called "More piezo stuff - high order networks, etc."