I have a similar setup, as I also run SD MixPre-D AES Out to the S/PDIF D. In on my Marantz PMD661 (OWM). If you only need the two channels, I would go with the MixPre-D and if you need more in the future, you can add a 302 or a 552 and link them to the MixPre-D as a larger mic/line field mixer. They seem to have dropped in pricing (used) with the SD663 on the market. Personally, I prefer and trust the A/D of the SD units and would prefer the MixPre-D, as it has been such an ease to use when I have done single channel Boom recordings.