On the receiver, all speakers should be set to small, with an 80hz crossover. The 800 crossover knob should be maxed out or on bypass to get it out of the way, since the receiver is controlling the bass management.
Are you using the subwoofer "pre out" on the back of the receiver, to the "input" on the sub, using cable like in the picture above, you mentioned a splitter, why would you need a splitter. Just a cable going from the receiver to the LFE Input on the sub with RCA connections.
M80s-VP180-QS8s-EP600-2xEP350 Denon3808 Outlaw7700