Let's see if I can help.
The RDA+ does not have the s/pdif connector included, but you can purchase it separately.
The RDA+ only has 3 connectors in the back for sound, but they can be programmed to suit your needs. The mic and line-in can be changed to center and rear channel to get full 6 channel sound.
The RDA+ does use the Nvidia sound found on the MCP-T. In fact, the driver interface looks exactly the same as the screenshots I've seen posted elsewhere on this forum for the Asus a7n8x.
No where in the documentation does it mention SoundStorm, but I think Hexus probably got it right. If an s/pdif is 'required' for certification, then this board wouldn't be 'certifiable' out of the box. Remember, SoundStorm doesn't MAKE anything, they just certify other people's equipment. There are no SoundStorm chips or drivers on ANY nForce2 board. Just some have been certified, and some have not. The sound processor is the same whether it is certified or not.
Hope that helps. 🙂