I have tried a bunch of separate USB/SPDIF converters before the AMR DP-777, including really expensive ones from Off-ramp with all possible options and modifications. Each sounds different but none presented a significant upgrade to running USB directly into AMR. That holds true until one day I inserted the Z-system RDP-1 before my USB/SPDIF dongle and AMR, it has brought the performance up to a new level, I would say night and day. Now I can never go back to the USB port running direct. Too bad I have to turn one four boxes (MacBookPro, USB/SPDIF dongle, Z-system RDP1, and AMR DP-777) before the AMR locks onto any signal, but it's worth it.
To all AMR DP-777 owners: If you don't have something like Z-system RDP-1 before your DAC, you don't know how your AMR is all about. It doesn't matter whether you are using PC, Mac, or CEC belt-driven transport. It doesn't matter whether you are using AQ Diamond or WW Platinum Starlight USB cable, they make 5-10% difference, but if you have the money, buy a good re-clocking device like the RDP-1 and sit in amazement! It's like transforming your $5K DAC to $10K, or from DP777 to CD77.