Auto is fine on the Slim. When watching BDs, the 519 can't decode the hi-rez True-HD/DTS-MA, therefore the Slim has to do it and send it to the 519 via PCM. For SD DVDs, the Slim could send a Bitstream and let the 519 do the decoding and the indicators would light up. You could try both ways to see which sounds better for DVDs.
For the other settings, need to know which speakers/sub?