apparently the pyle unit has 4 independent channels, so i could run the subs by themselves. but you say "mains". i dont have mains. its basically a 3-way system. i am turning my full range Bose speakers (2 drivers in each cabinet, a 10" woofer and a 1" soft dome tweeter) into the mid/high speakers, and then the 18" driver the low woofer. so i figure i do -3db choke on the 18's around the 100-150Hz mark, then bandpass the woofer on the bose so it doesnt get crushed, etc.
so in essence, i could run it all from one set of amp outputs, but i can indeed run the 18's off of the 2nd set of outputs.