The WU-SPK wiring upgrade includes a pair of NL4 SpeakOn connectors with four NL2 SpeakOn cables on the interior of the cart. This upgrade is for audio systems that use passive loudspeaker and ideal for sending separate signals for full-range speakers and subwoofers through a single 4-pole SpeakOn cable. 
 
This upgrade is most often used with mixer carts, but it is available for synth carts as well.