Active circuits are able to filter, attenuate or boost the signal from the pickup. The main disadvantage of an active system is requirement of a battery power source to operate the preamp circuitry. Batteries limit circuit design and functionality, in addition to being inconvenient to the musician. I guess this make the preamp character for the piezo sounds smooth in treble area. BTW I succesfully made the circuit for the owner of this preamp.
B: Having much greater preamp headroom, and bypassing the R91's mic preamp altogether, using a hi-fi or low noise op-amp to go straight into the line input. Design Considerations. The circuit needs to bring the signal from an electret microphone up to a level suitable for a line input.