You leave the plastic cap on to reduce the power. The impedance of the ultrasonic depends on the frequency. Piezo speakers are capacitive. The capacitance of the Spooky2 Central peizo speaker is 1500pF.
The generator can drive the speaker directly because it is protected against overcurrents.
We have tested the piezo being driven by the XM generator directly with no amplifier. The power is reduced but it still works OK.
For more details, please see this blog: