|They make a beautiful couple...|
HobbyPCB sells a "KX2/KX3 Interface Kit" that allows either of those rigs to be mated to the Hardrock 50 in such a way as to be able to allow the transceiver to control automatic band changes made to the Hardrock.
The interface kit is nothing more than a breakout board for the Hardrock's serial connector, located on the rear panel, and comes with the required cables - three for the KX3 and only one required for the KX2. Both the rig and the amplifier need to be configured in order to communicate with each other; this is a simple procedure described in the video below:
As can be seen (and heard) the amp performs quite well with the KX2. QSK keying with the amp is identical to QSK when using the KX2 alone.
I don't yet have the built-in autotuner installed but will get to it in the next 2 weeks or so. In addition to adding autotuner capabilities to the amp, it also adds 60m functionality.