Is it possible to drive a power piezoelectric in lower frequency than its own frequency?
If you don't care about maximizing the output amplitude when lightly damped then you can operate away from mechanical resonance. 

Operating a resonant transducer at it's mechanical resonant frequency allows relatively small drive voltage and current to result in a lot of motion (strain). 

Efficiency aside, since there are limits to drive voltage, whether it's supplied by a coil or an amplifier, operating away from resonance limits the maximum output amplitude. From this website.