Ethernet Cables, do they make a difference?

You could do that. But why not swap cables in someone's personal system that he/she's intimately familiar with?

This black box would sit in the persons system. They would provide the USB cable on back. 

@markalarsen 
Look... I understand the only way for you to contest the technical evidence with your level of technical ignorance is to bully and abuse and call me names. I don't take it personally. You went out to the internet, copied and pasted some obvious nonsense you didn't understand, and I took you to task on it with FACTS. Facts, by the way, you don't have the technical knowledge to comprehend. Truth be told, I don't have the piece of equipment you're talking about. I'm working on putting together my solution to that. It's looking like it's going to be a Raspberry Pi-based solution. There's some interesting DAC hats out there I'd like to give a try. One thing is certain though... When I come up with it, it most certainly won't depend on Ethernet cables.
You see, I don't have buckets of disposable income. I'm a middle class single dad trying to put his daughter through a private boarding school to the tune of $25,000 a year, with some significant help from some close friends. Aside from that, I'm figuring out how to represent myself in a rather complicated legal case. Since I'm absolutely dominating in the case, I'm thinking I'm figuring it out, with the help of some very good lawyers kind enough to give me free advice.
I get to listen as well as I can figure out how to build, rebuild, and modify. My Focals are the only real big investment. I take a LOT of BS from clowns around here for my analytical approach to evaluating claims. I don't have the cash to waste putting faith in the marketing department's ad copy or some "designer's" unsubstantiated claims. Therefore, "show me the measurements!". I figured out a long time ago that I can make up for what I lack in funds with learning and doing. So that's what I do. Personally, I think an excess of cash makes for a real fool when it comes to understanding in a wide variety of endeavors, and most especially this one. I'm genuinely shocked sometimes at the amount of money people spend on things they don't understand only to be confounded when it doesn't operate as they believe it should. Cars. Computers. Stereos. TV's. I do feel safe in saying I have a grasp on the basic technical principles of everything in my home. There's nothing here I would be afraid to open up, take apart, and reassemble. The HVAC, my car, the refrigerator, the washer or dryer, the computer I'm typing this on, my cell phone, or my stereo. The real irony is that most of you probably have owned or used something made with the tooling I produced and/or inspected. Ford, Chysler, or GM cars, Price-Phister faucets. Harley Davidson motorcycles. Pratt&Whitney jet engines, Textron golf carts, to name a few. But I'm the guy with NO technical knowledge on anything. 

Whatever.

nonoise
“I found this statement from John Atkinson (the one that measurement obsessed folk fall back on for support) to put a kibosh on most A/B/X tests:

...2nd order behaviors -eg-distortion signatures and noise modulations that characterize sound quality, and these things take a lot of listening to identify.

It’s what I and others have repeatedly stated: long term listening is the correct way to identify sound signatures and clues. You can get the gist of a cable or fuse or what have you but it takes immersion in the music, over time, to correctly assess anything, unless it’s something glaringly apparent.”

..........................

>>>>>I have serious problems with long term listening as a definitive method of testing. And I’m going to tell you why. First, things change over time for any number of reasons, even day to day and hour to hour the sound can change on a whim, for no apparent reason. But sometimes the listener does make actual physical changes to his system during the long term listening. And those changes, conscious or not, can affect the sound. So, which is it, the sound on Sunday morning at 7 am or the sound on Thursday afternoon at 3 pm? Second, audiophiles frequently complain of audio memory being an issue, so what’s to prevent audio memory from being an issue in long term listening?

Once again it looks like we’re faced with the likelihood that there is no such thing as a scientifically valid listening test. There are too many variables in the case of long term listening to be able to definitively conclude anything about the device under test. If someone can’t hear it in a short term test there’s no guarantee he can hear it in a long term test.