SPDIF/TOSLINK digital audio protocols are widely used in AV systems. From your digital audio streamer to your bluray/TV/Set top box, some might believe that SPDIF = SPDIF. That's unfortunately not the case hence the need for this short summary. There are two types of digital audio data to be streamed over SPDIF (Coxial RCA to RCA cable) or TOSLINK (Optical fiber) connectivity. 

- Stereo/PCM audio: The most common stream from a CD player/Network audio streamer/your DAC. The SPDIF/TOSLINK stream is a two channels of uncompressed audio for up to 24bit. Little known to most consumers is that most devices are actually outputting 20bit. Stereo PCM audio is the protocol supported by all miniDSP platforms. 

- Encoded bitstream for multichannel audio (5.1/7.1): Very common for your TV/Bluray player, algorithms such as Dolby/DTS "bitstream" encode the audio to carry more channels using the same bandwidth. For a device to be able to understand a bitstream, it needs a decoder. (e.g. Dolby/DTS decoder). miniDSP devices won't support such multichannel SPDIF/TOSLINK stream. 

To sum it up: You can only interface stereo audio stream to your miniDSP device. Make sure you set your source to be Stereo PCM. If your source only supports multichannel, you will need a Dolby/DTS decoder (e.g. AV receiver) in your chain.