My understanding is that HD audio formats cannot be passed over the older SPDIF or Optical digital standard - not sure if this is a bandwidth limitation or one that is forced by lack of encryption support, either way it does not work.
Options:
1. Your player bitstreams (i.e. native DD or DTS) and sends to SP2 which handles decoding. Only applies to DD / DTS.
2. Your player decodes and sends as PCM to SP2. Only applies to DD / DTS.
3. Your player decodes, converts to analog and sends analog to SP2. Can apply to any format, inc HD formats. Can't recall if SP2 has 7.1 or 5.1 analog inputs
4. Your player decodes HD audio, downsamples and sends to SP2, or only uses the DD / DTS part of the HD stream.
It looks like you are seeing option 4. You should be able to tell by reading the display on the SP2 what type of signal it is receiving.
Which will sound best? Trial and error, I'm afraid, too many variables like quality of decoding and AD conversion in the bluray player vs quality in the SP2. Can also depend on SPDIF vs Optical, and ultimately on your systems ability (or your own ability) to resolve any differences in the sound.