I use the "Y" cable with the KPA500 AUX cable to connect the KPA500 and the PR6 6m preamp control line to the K3. The RX passthrough for the PR6 to use RX only antennas doesn't work when the KPA500 is connected, powered on or off. Searching the reflector archives, I was relieved to find that I was not alone and that this problem had been identified.
The connection of the KPA500 to pin 11 changes the threshold that triggers the PR6 on/off relay so that the PR6 remains on (and hence no passthrough function) no matter what the menu setting of DIGIOUT1 on the K3.
Since the pin 11 inhibit function of the KPA500 isn't used at the moment, the "fix" is to open pin 11 in the AUX cable. Rather than damage the cable, I used HD15 M/M and F/F gender changers (I could not find a M/F locally) to create the equivalent of an HD 15 M/F gender changer. I twisted off the male pin 11 creating a pin 11 interrupter essentially the same as the pin 10 interrupter Elecraft supplies with the AUX cable.
The PR6 passthrough function and the DIGIOUT1 setting now work correctly.