@azwill I bought my unit from an authorized dealer in the US. My source (Aurender A10) is definitely a weakness. The A10 is one of Aurender's earlier units and its USB out is relatively not great (A10 was designed more as a DAC than streamer for outboard DAC). However, I have a DDC that in the chain that cleans up signal before going to DAC. I have used the following DACs while/since owning the D10x (Topping D90, Gustard x26, Holo Audio May, Lumin D2, T+A DAC 200); I would choose any of those DACs over the Luxman. Similar to your experience, I2S has been the best connection in my experience (a connection the D10x lacks). The USB input on the D10x is not very good either; its coaxial input has a much better sound but can't accept DSD signal.
Newer DACs like the Holo May and T+A 200, do such a good job cleaning up incoming USB signals that a DDC is redundant with those units. The T+A is the most revealing DAC I have ever heard in my system; On one of my favorite jazz albums (Woody Shaw – In my own sweet way), I was able to pick up the minute vibrations and not just shimmer of the hi hats (I have never experienced that level of inner detail from a DAC or turntable). The DAC 200 also outpaced my Holo May with respect to bass and dynamics. It was almost shocking what that little box could deliver.
I decided not to keep the Total DAC and continue with the Holo May, because I could still pick up just a touch of that typical Delta Sigma sound signature (hints of glassiness and coldness that affects musicality). The T+A let me know what more was possible as you move up the ladder, something I never got from the much pricier D10x.