Best answer by Stanley_4
If that gets you no sound or activity then move the sub RCA to one of the non-sub Pre-out ports (you may need to adjust the volume up a bit) and see if it goes active, if it doesn't hook a set of headphones to the tape-out and make sure there is sound present there.
If it doesn't go active with sound present on the tape out ports it likely you have an issue with the sub setup in the Denon. Grab the manual and sit down with it and try to work through the setup. If that Denon offers a web interface it is usually a lot easier to use than the on device display, even the TV out option beats that.