I use the MoXF8 on Cubase too. Please clarify the following:
(1) Is your SX900 connected to your computer/laptop or to the soundcard directly?
(2) Is the SX900 connected using a USB cable or a midi cable?
TROUBLESHOOTING
(1) Please connect your keyboard to the computer using USB cable (not to the soundcard)
(2) Check on the inspector routing section if Cubase recognizes the inputs from your keyboard (in my case it says MOXF8 in 1 to 8…there are up to 8 inputs. I usually route to 1 to get sound)
That should be all you need. No need to configure anything anywhere else…that would be overkill.
Let me know if this is of any help to you.
Goodluck