Great question @p7775! The Doorbell Pro does not need a wire wound resistor, as that part is only required for our Battery powered devices. As the battery devices operate off the battery even when hardwired, a resistor is needed to regulate how much voltage reaches the battery powered Doorbell models for a charge and use of the existing chime kit.
The Pro, Pro Power Kit, and plug-in adapter all feature a power regulation of some sort. As The Pro is rated and designed for a continuous source of voltage and operates solely off power, it is able to be installed to a 16 - 24VAC power supply without any resistor. Also, if you are using a plug-in adapter, you will not need to use the Pro Power Kit. I hope this helps! 