I'd like to summarize my point of view and which conclusions possibly to draw and which ones certainly not to draw.
Based on what I've written regarding the choice of power supply for the D50, everybody is invited to do the same low cost testing. Even if I don't think anybody will come to the exact same findings than me, I'm confident that anybody will detect the difference in overall SQ.
The sub-par power rejection of the Topping D50 puts the customer in a situation, that its onto him who has to find out among a myriad of possibilities what may suit him best.
The same holds true for the sub-par USB input of the Topping D50.
Even worse, there simply is no straight forward way to predict what may or may not suit you, not even if cost is no object or for those already knowing quite a lot about sonic impact of specific settings, or the ones that read tons of silly reviews or this silly sentences I put up here for the matter.
Going the "science" route by measurements, does not help either.
Remember the D50 measures great and the pure battery power supply I have tested measures even better, so measurements are of little help when it comes to sonic signatures.
Same holds true for any "cleaning" attempt of the USB signal, as my experiments with the Intona USB isolator undoubtedly allowed my ear brain system to retrieve quite an increase on details and dynamics, but also puts my nose right on the sonic signature of the Topping D50, which I, for one, didn't find any favourable.
On top of that, my PSU measurements shown, do only cover out of audio band behaviour, which "should" not be any cumbersum, and further on, do tell us next to nothing, as to why the el cheapo HAMA hub for example provides pretty well defined bass.
Also those plots only show, what is embossed on the D50 by the power supply, and by no means how the power supply reacts on transient current demand by the D50, which in fact is a completely different story.
So, is the Topping D50 a good buy in the long run?
It depends, I'd say.
If you are lucky with what you've got, enjoy!