I updated the BIOS to the latest version, and got myself an optical audio cable.
Using a plain and normal audio cable for the main audio out on the motherboard to the audio in on the 6fire is working for the audio.
Using an optical out from the motherboard to the optical in at the 6fire is also working very well - as for both with the ASIO through the USB connection too.
Audio through USB is still not at all working.