Since you are ripping with the DTS HD audio track, it contains lossy DTS core audio as well which means that you should be able to bitstream that portion through S/PDIF. Since you are able to get stereo sound, it seems that what you are doing is decoding the audio to PCM and then sending via S/PDIF.
It has been a while since I have used FFDShow Audio, but IIRC, there is an output section where you can configure audio passthrough types your AVR can accept. Are AC3 and DTS selected there?
