John,

AVR's with one Subwoofer output (Black Jack) can utilize a single RCA Cable to the Sub-woofer. However, most active subs have both Left & Right Channel audio Inputs so a Y-Female In to Twin Male RCA Out adapter cable is required. Alternately, you can utilize a Male Y cable, at the Denon AVR, and then a stereo RCA from there to your Subs stereo inputs.

The Denon AVR Bass should be configured as "LFE + Main" in the speaker setup section and most system the crossover should be set to 80Hz