Hi Tuomo,
you should have received an Owner's Manual with the KS-SW100 subwoofer. Please read this manual first. If you don't have this manual you can download it here:
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https://fi.yamaha.com/fi/support/manuals/index.html?l=fi&c=&k=ks-sw100As mentioned in the manual, you simply have to use the included "RCA to 6.3 mm Phone plug" cable to connect the RCA input socket of the KS-SW100 to the "
Main Out L/L+R" socket of the PSR-SX900. That's all.
Make sure that the subwoofer is switched OFF and the volume is set to zero. Connect the power cable to the wall socket and the Audio cable, as mentioned to the SX900's Main Out mono socket.
If not done so, first switch on the SX900,
then switch on the subwoofer. Set the Master Volume on the SX900 as desired and play e.g. a Style. NOW increase the Volume of the subwoofer to add the sub frequencies as desired. (Since the Main Out signal of the SX900 depends on the position of the Master Volume dial, you can now use it to regulate the overall volume.)
P.S.
If not done so, I recommend you to download also all the manuals of the SX900 (at least Owner's Manual, Reference Manual and Data List):
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https://fi.yamaha.com/fi/products/musical_instruments/keyboards/arranger_workstations/psr-sx900/downloads.html#product-tabsHope this helps!
Best regards,
Chris