Play the CD through a digital input. I have both normal and inverted inputs on my sub. With the digital crossover engaged, and playing that particular test tone freq, I pick the sub input with the loudest response at the listening position. (I also adjust the variable phase knob for max output too.) 5 - 10 Hz above and below the crossover freq yields the same result.

Then, I use the analog outputs from the player (player: all large; sub on, but doesn't even matter for this test), into the front 2 channels of the 5.1 inputs. I engage the 80 Hz analog crossover on the 950. I redo all the optimization for max output. In the end, simply switching sub inputs yields the max output in this case.