I'm confused. Let me rephrase that:
Receiver:
no sub; LFE + Bass to Fronts; Fronts and Center set to Large; Rears set to small (?).
Receiver to DCX:
pre-outs L, C, R to DCX inputs A, B, C (respectively).
DCX:
Sum -> A+B+C
Outputs:
1 - Left Channel High passed (at the crossover point) - input = A
2 - Center Channel High passed (at the crossover point) - input = B
3 - Right Channel High passed (at the crossover point) - input = C
4 - Sub Channel Low passed (at the crossover point) - input = SUM
The exacts filters will depend on your room. I'll let others explain that