You are right Eric, there is nothing to worry about the signal, the interface would just work as a digital I/O for the computer. The only thing that you have to worry is that the pll of the interface locks to the output of your A/D and that your D/A locks to the output of your AES interface. (Assuming the A/D and the D/A are separate boxes). Well, not quite, another issue is how stable the interface driver runs in your computer and OS. For that matter I would recommend the MOTU 896HD, it has AES I/O, even though you would be paying for much more features than you need, but the drivers are very solid for OS X. Also the Metric Halo 2882 has firewire to AES but also has other type of I/O so it is the same as the 896 in terms of paying for things you don?t need.
Good luck
Hans Mues