again with the USB and the evil sources :'(
last time I tried giving my opinion by talking about how stuff work, but nobody cares about technical reasons (else I wouldn't see so many people with NOS DACs ^_^).
so maybe I can try basic human logic this time?
whatever is in a magic USB box or any crappy hub with an external power supply, it's made from available and relatively cheap components, and however you wanna see it, those components would work the same way if they were already inside a DAC. so why wouldn't your favorite DAC be already equipped with whatever is in the magic box to make the audio "better"?
to that I can see a few very logical options:
-1/ the manufacturers did just that, the DAC already has external power supply, super clock, and whatever trick available.
= you're just adding redundant products in the signal path and probably making the signal worst.
-2/ the manufacturers of that DAC don't know what they're doing. they added USB input because people kept bothering them for it, but don't know much about USB and made a poor design.
= you bought the wrong DAC or at least you shouldn't use the USB input.
-3/ they know but decided to save money instead of using 40$ of components to make a clean USB signal and improve the DAC specs.
if it's a cheap product, well why not. but if it's some hifi stuff, you've been swindled.
-4/ they know and tested all configurations to realize it was making no significant improvement. so they decided to not add useless junk.
if they get great measured specs, then they were right not to bother. else goto -2/
so from this very basic manufacturer's side thinking, and I don't think I went wild with imagination here, the only time where adding something to the USB cable might improve things will be with -2/. and even in that situation you have to take your time and think if you should go for another input, go for another DAC, or if you really want to keep this one DAC with a crappy usb input? then and only then, look for a magic box or a powered hub, or update your usb drivers on your computer or get a raspberry pi as dedicated audio source...
that's why I do believe that when a DAC has troubles with some source, most of the time the answer is to get a proper DAC instead of trying to patch things up. (and by proper I don't mean expensive, I mean proper for your own needs).