I've got a MOTIF XS-8 keyboard with on-board Firewire interface which works well under WIN10 connected via a $25 SY-PEX30016 PCI Firewire card with a TI chipset:
http://www.newegg.com/Product/Product.aspx?Item=N82E16815124107&cm_re=SY-PEX30016-_-15-124-107-_-Product .
I don't use it for recording much since I prefer my keeping my ASIO USB3 interface available in SONAR, but I like to test and play with it to make sure it does work if I need clean multitrack audio out of the MOTIF from time to time. It seems to work well with the latest drivers from the Yamaha download site noted above.
Are you setting up your outs correctly on the MOTIF? Just because the computer chirps when it sees the MOTIF doesn't mean the MOTIF is set up to send audio via the FW. The manual is a bit confusing at times when it uses the "mLAN" terminology when it is really referring to the Firewire interface. However, follow these instructions and substitute Firewire for mLan:
"Make sure that the mLAN connector of the MOTIF XS is enabled for audio communication. When transmitting audio signals via the mLAN connector, set the Output Select parameter of the MOTIF XS. When receiving audio signals via the mLAN connector, set the mLAN audio input part parameters of the MOTIF XS. For details, see page 45. Press the [STORE] button to store this setting. . . ." Otherwise, not sure why it isn't showing up for you as a sound device in WIN10 or SONAR. It may be that the MOBO firewire interface isn't playing nicely. You could buy and try the TI PCI card above and return if it doesn't solve things.
No other ideas. Good luck.