The effects knob has nothing to do with the output to the mixer.
First. set each channel of the mixer to USB for the input.
Next, open Ableton Live and assign the Audio Output Device to be the Pioneer DJM-900nexus. Then, click the Output Config button and make sure that only the STEREO PAIRS are checked (un-highlight the mono channel outputs).
Close the settings and find the four audio / MIDI channels you want to output, click the I-O button (over by the master) if you don't already have that portion of your channel-strip showing, then click the drop-down under "Audio To" (it defaults to MASTER), and change it to EXT. OUT. Finally, in the drop-down below that, you'll see a pair of outputs - select the pair you want and you're off to the races.