@russbutton
I’m not familiar with the D&D unit you’re referring to. But, it does seem like such a unit attempts to be a one box solution that cuts out the variables. The only thing I think you’re missing is that each component can be optimized for audio quality and there are necessary limitations to any one box solution whether that be in design topology, quality of parts, power supply(ies) and isolation. But, $14k all in is an attractive price point relative to what I have invested in my digital system.
If I read correctly, you’re still understandably skeptical of the source’s value in the equation and that because of your location, are unlikely to experience worthwhile alternatives. To that I say you’re fortunate as you won’t fall down the proverbial rabbit hole.
You’ve obviously spent more effort and $ on your vinyl rig than on your digital system - which I get. I used to have a vinyl rig and miss it at times. But it doesn’t take a lot of money to make notable improvements in even modest digital rigs.
As I experimented, I began to hear, and understand the benefit of optimizing all the disparate digital gear in my system. It is more than just 1s and 0s. As already discussed - files are being transferred bit perfect. But, the noise that’s carried along the same wire along with the slope of the voltage changes which the DAC reads as a 1 or 0 are critically important components and are plainly audible in direct comparison. The human ear can detect mere femto seconds of jitter. It is very well documented (in listening tests and actual measurements) the impact of incorrect impedance of a digital cable and the length of the cable.
The OP started this thread wondering about the merits of separate components and it has been a good discussion and one I hope helpful to you - a skeptic. Being a skeptic is to be encouraged in audio as there are plenty of ways to separate people from their money. But, please keep an open mind and seek direct experience to weigh against the theoretical.
Best,