I prefer using the sub output from the receiver to the RCA jack on the sub woofer.
And the method that Polk is recommending as the second method.
If you use this method you will have to set your Onkyo to a 5.0 speaker system and you front left and right to large. I looked at the owners manual this sub is not designed for a sub output from a receiver.
Options besides the ones in the owners manual. This sub unit has RCA inputs, run a coax audio cable from the sub output on the receiver and get a RCA "Y" splitter and feed both L and R RCA inputs on the sub. Set the crossover frequency on the sub to 160hz and the crossover frequency in the Onkyo to 80hz. John
As for x-over frequencies 80~100Hz is the best range.