Optical Cable from the back of the PC to the amplifier
Figured that before I read the thread:
Optical can carry one of three audio formats:
5.1 Dolby Digital
All PC audio is uncompressed PCM, which optical can NOT transmit as 5.1 over optical. Only Dolby/DTS tracks can be passed through as 5.1 via optical [which is why, if you watched a movie with a DD/DTS track, you'd get 5.1 in that movie, but not in games].
Your only option to get 5.1 out of the optical port for games is to buy a soundcard with a realtime encoder to either Dolby or DTS [which most do, for this very reason].
Also FYI, the only thing "speaker fill" does is repeat the front channels onto the side/rear speakers; it doesn't give 5.1, just copies the stereo signal to the other speakers.