It depends on how its drivers present the multiple outputs, ie. whether the multiple outputs are presented as a single 8 channel device, or 4 separate stereo devices. For those 2 scenarios, checkout the SPEAKERS and MULTI examples, respectively.
As it has ASIO support, you could also use BASSASIO
(checkout the included SPEAKERS example).
Are there any examples out there on how to create the file(s) with 8 different soundtracks and push it through bass to the device's 8 channels?
Do you mean combining multiple existing files to make an 8 channel file? If so, you probably don't need to bother, as you can choose which of the device's channels to play the BASS channels on (eg. using the SPEAKER flags).