I don't have "K" speakers myself. My speakers dont have built-in crossovers so I've been using aux 6 for the subs and the 31 band GEQs as crappy crossovers - mostly so I don't have to bring an external crossover.
I will tell you, there is definitely something about having a matched system. At first I thought that the crossovers on the "K" series were just average, but I was wrong, especially since the KSub and K12/10/8's were matched. I would assume the cutoff's on the K12/10/8's crossovers are as follows:
EXT SUB = 100Hz
Normal = 80Hz
Deep = Between 50-60Hz (K12 rated at 48Hz to 20KHz)