Media Player Classic Downmixing Surround Sound to Stereo via S/PDIF Output

Hey guys - so I just got a new sound system that is connected via s/pdif optical and it can only decode Dolby Digital 5.1 surround.

I started using Media Player Classic and set it up so non-Dolby Digital sources can be played (via selecting only Dolby Digital in options>>internal filters>>audio decoder).

My DTS 5.1 sources are now playing on my system, but I noticed they are being downmixed to stereo. Is there any setting to force surround sound from non-Dolby 5.1 sources or to stop them from being downmixed to stereo?