Here's a good tech document on interfacing;
RaneNote INTERFACING AES3 & S/PDIF.pdf

I have used a 518 and a CO3, both are 'bit perfect' in use, I also used a Behringer DEQ2496 to convert optical to AES, it was also bit perfect after making sure everything was switched to the right settings.

518 has limits, but does interface with other Meridian equipment very well.
The 618 will also do what you want, but are quite rare.

All the best,

Steve.