The manual for Yamaha's KS-SW100 subwoofer shows it connected to the L/L+R output jack.

Strangely the Yamaha UK website shows a picture of this subwoofer connected to a SX900 but the haven't updated the rest of the web page to add the SX keyboards to the list off comparable keyboards (even though we know they are). The EQ settings shown just look to take a little bass off the keyboard.