Not a dumb question at all.
The cable set pair is part of six cables I built yesterday specifically to allow various combinations of send and receive in both balanced (XLR) and unbalanced (RCA) to and from the 2i2v2 (or the other interfaces). As it has 6.35mm and XLR in as well as 6.35mm TRS out. I wanted the full swing to be available if I needed it at each end, but to also allow chassis reference shielding on the DUT if it was unearthed. (otherwise you essentially short the cold to earth (IIRC via an internal 100R res) and can get distortion)
Complicated, yes. But all my test gear sits on a common ground/safety earth. The 2i2v2 is also sitting at the same potential via the USB connection to the PC. Yes, I could isolate it, but the second I connect a 'scope or other piece of proper earthed test gear, I can kill half the swing. It's better to stick to a common ground and make the differential stuff to suit.
If I go portable (laptop on battery) the interface can 'float' with respect to the DUT, even if it is safety earthed or not. Makes a massive difference (>10-20dB) in the low end mains and harmonics.
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Original 1980s NOS metal made in Japan RCAs. Down to my last few. Core/hot goes right to the tip and soldered at the very end. Best way to do it IMO.
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