Most apps Spotify will reference the device set as Default Device for Windows Sound Playback Devices. Other programs may have their own device and I/O settings.
The SL16.0.2 must be installed with Universal Control classic.
You can download here.
In Universal Control, in the Setting menu, select WDM setup. This window allows you to assign 8 WDM streams to available input and output channels of the mixer.
You still need to mix the firewire returns on the mixer.