Optical (and coaxial) can only carry a Dolby Digital or DTS 5.1 bitstream or PCM stereo. The BD50 can decode TrueHD, DD+, and DTS-HD (including Master Audio), but the only way to deliver that to the Model 990 is via multichannel analog cables. You will want to connect the BD50's multichannel analog output to the 990's 7.1 Direct input, as this will give you access to TrueHD, DD+, DTS-HD, and the multichannel PCM tracks provided on many Blu-ray discs.