I think I confused you when I started talking about the HDMI converter, sorry about that. Plus you need to know a bit about both SPDIF and HDMI circuiting, digital converters (chip), so on.
Here is the ALC 889 (my old mobo), at the bottom you see the basic outlay of the SPDIF circuit after HDA. The digital converter is 2 channels only, same on input.
The Z906 will have the same, on its input path, a 2 channel (PCM) digital converter, had it been a 6 channel one, I could receive 6 channel PCM.
Officially my HDMI converter supports LPCM 5.1 over optical, but after talking to the OEM 8 channel was also confirmed.
Some (although very rare) receivers do actually support 5.1 LPCM on optical, but rarely.
----
To validate, check your Realtek optical has [24bit, 192k x 2] channel support (mine does, Z906 also accepts 176k), that is 9.216 mbps and 384 total samples.
Now lets work within that limit (same as HDMI doing 32 channels over 1536 aggregate (total samples)):
[24bit, 48k x 8] channels, that is 9.216 mbps and 384 total samples.
--
Also note, 9.2 mpbs (based on digital converter) is enough to bitstream all but Dolby TrueHD and DTS-HD, based on the standard format list.
====
My newer mobo with S1220-A can do 32bit on analogue out, and my Z906 has no issues with that either, pretty decent IMO.
176k (edit) on SPDIF and 32 bit analogue, shame about the 2 channel digital converter on SPDIF.
Link:
Logitech Z906 5.1 Surround Sound Speakers System