Sound problem with Blu-Ray player/receiver

Hey all, I have no sound from my subwoofer with blu-ray discs in my new Sony BDP-S570 and I'm hoping someone can guide me to the right setting.

Blu-Ray player: Sony BDP-S570 with latest firmware
Reciever is an older Onkyo HT-R510

Audio is connected via Digital Optical (no HDMI with this receiver)
Running a 5.1 setup

Blu-Ray Audio Settings are:
BD Audio Mix: on
Dolby Digial Coaxil/Opital: Dolby Digital
DTS (Coaxil/Optial): DTS
Audio DRC: Auto
Downmix: Srround

When I run the audio optimizer that comes with the Standard Def DVDs of Toy Story 2 and UP in the blu-ray player, the sub comes through loud and clear (no sound comes out of the center surround speaker, as there is none).

When I run the blu-ray versions of the audio optimizer on those same two movies, I get no sound from the sub. However, when the "center rear surround" test runs, sound comes out of both left and right surround speakers for some reason.

Any ideas? Thanks for the help!