I have just purchased a D50 as an replacement and upgrade for my broken H4n. I have first considered using a Sound Devices MixPre D as front end going into the digital in of the D50, too. But after some research on the net I have learned that:
a) this would require using a separate box to get the signal from the AES out of the MixPre into the SPDIF in of the D50; if it was a coaxial SPDIF in on the D50 the above mentioned cable with matching resistance would be fine, but unfortunately the D50 has an optical in ...
b) most likely the Sony would not be able to handle the digital signal of the MixPre even with a proper physical connection; I have read several negative reports on this, which is caused apparently by the Sony expecting any digital signal coming in to be in a specific data format (containing bit depth and sample rate in the header) which many digital devices do not deliver, including the MixPreD. Sound Devices kind of confirmed this issue in their user forum, and recommended the USBPre2 instead as a front end, as this device is able to deliver the required digital data format (and it has optical outs). They might consider updating the firmware of the MixPreD as well some day, but it didn't sound as if you should hold your breath for it.
This is all based on internet research and hearsay though without any hands on experience, so maybe somebody managed to use a MixPreD as digital front end for a D50 and can share his experience.