I have a similar board (ASrock P55 Pro) with the same codec (ALC890). I tried your DSDT and legacyHDA.kext file. The sound works from both my SPDIF outputs (optical and coax). It sounds distorted through the regular 1/8" headphone jack though.

Did you follow a guide, or rather, could you explain what you had to modify in your original DSDT? I want to perfectly adapt it to my board/DSDT.

Thanks!