The Sony BDP-S5100 will play regular CD's via PCM but not SACD's via DSD but I thought it would convert to PCM. It did with my 300 but not my 510 and I have not changed in settings in my Sony BD player so it has to be a setting in the 510. I've read the manual but so far I haven't found any info.
Works fine with an OPPO player. There really aren't any settings you can get wrong on the MRX for this. But check the Input definition for that Input and make sure it is really set to use the HDMI Audio Input -- and not, say, Optical or Coax.
Check what your Sony manual says about its SACD playback. It could be that it doesn't really play SACD at all (even as PCM) but DOES play the CD-style layer on a "hybrid" SACD disc. Not all SACD disc titles are "hybrid" so it may be the discs you tried before just happened to be so.