I have a Denon 3310 hooked up to a PL200.
On the PL200, the Crossover is set to OUT, right?
OUT means you are NOT using the Sub's internal crossover. It is the same as BYPASS.
When you play something, look on the Denon LCD screen to the right side. It shows the output channels.
If 2.1, it should show 3 things: Front Left, Front Right, and SW.
Does it show the "SW" on the Denon LCD?
1) As already mentioned, in your setup, make sure you have the Sub configured in the speakers setup as YES. All other speakers Small, XO 80Hz.
2) The output in the back of the Denon labeled "SW" is the correct one. It should be next to the other pre-outs (FL, FR, C, SR, SL, etc.).
3) If listening to 2.1 music, set up the Audio Stereo/Direct as Subwoofer YES, speakers Small, XO @ 80Hz.