You say "A" lightpipe cable- You are aware you need 2 (both in and out) if you want to sync the digimax to the hdsp?
BTW as said before differences are minimal, but there is one simple rule for best results. When recording set the device you are recording with (analog ins) as master. When you do critical listening, set the device you are listening through as master (analog outs).
All recent devices run best at their internal clock. Even if an external clock is much better then the internal one.
If you need an external clock for other reasons (complex multi device setups with video synced too) It is of course best to get a good external clock.
If you want more details, there has been a great article in SOS on this topic.
Vincent, Amsterdam
https://soundcloud.com/thesecretworld
BFpro fs, 2X HDSP9652 ADI-8AE, 2X HDSP9632