I heard my name was called.
It comes down to whether you want the headunit or KX3 to manipulate the sound. Typically, outboard crossover has a larger range of adjustment and makes it easier to see what frequency settings you are using. I've installed the KX3 before, and it's a simple, but nice piece.
With the KX3, you can actually control the sub channel output by adding a remote gain control, this is like having the 9815 sub channel volume control. I prefer having an outboard crossover to fine tune the frequency spectrum rather than the headunit. So I would run the F and R from the 9815 to the inputs of the KX3, and then the three outs to the amps.