Bruce:
When I first got my DDA-100, it was in my main system and I ran it exactly as you described - Oppo BDP-103 to DDA-100 via coax, using USB drives and occasionally the Oppo's disc transport. Worked great and sounded very good. It's now in my office system, running USB from my computer.
I frequently go through stages in my audio gear thoughts, vacillating between more expensive/complex changes to my system vs. less expensive/simpler. I'm going through one of these periods right now and am seriously thinking that having a DDA-100 in my main system, with the right set of speakers, is the way to go. Really simplifies things and can be VERY satisfying with the right speaker match. Something very gratifying about a simple, effective, enjoyable solution - who needs 20 boxes, 10 sets of interconnects and all that $$$ spent. Who knows where I'll land.
-Mike