the receiver is stuck saying "PCM 96" on the display and plays STEREO
This is a limitation of PCM over S/PDIF. It does not have enough bandwidth to carry 96 bit signals with more than 2 channels. It may need to be converted to a 48 bit stream by AC3Filter and/or output as a DD or DTS bitstream.
Not sure what the program source is. If it's Blu-ray, the only way to get full multichannel Dolby TrueHD or DTS-HD output is over HDMI using bitstream. Conventional DD or DTS must be used for S/PDIF. If its available, DTS will often sound better (depending on how it's encoded.) Most Blu-ray disc include a DTS and/or DD stream.
As mentioned by four, GraphEdit is a good diagnostic tool. Note that AC3Filter need not be used all the time. It's only needed when a another suitable codec is not available. If the use of AC3Filter is preferred, check the "Prefer AC3Filter" under "Filter Merit" in the AC3Filter options. (See graphic above.)