Done end-to-end, including between components, and assuming the components are fully differential internally, I have no doubt there's often a measurable positive difference in signal quality due to CMMR. Is it audible? Is the measurable difference worth the added cost in circuitry and cables? I don't know the answer to either one of those questions. I do know that a lot of people who really don't know what they're talking about think the terms "differential", "fully differential", "balanced", "quad balanced", "dual differential", etc., sound cool, sound special, and sound like they must make an audible difference in component design. Add in XLR connectors with their big metal construction and heavy weight, and you have an instant recipe for audiophile or HT nerd preference.
- I have said many times that, for consumer audio, except in the rare case of a high-noise environment, the biggest benefit is the ability to eliminate a ground loop that is quite audible (and annoying). That is the main reason my rear subs use XLR runs from my AVP.
- CMRR may be a consideration if routing long runs through walls near power lines or bundled with power cords behind a rack or console (typically for aesthetics).
- You gain CMR for all the schemes, but it ranges from very low (typically 6 dB'ish for impedance matching) to pretty high (60 to 80 dB for well-designed differential links).
- I do know what those terms mean, at least technically (from an engineer's point of view). So do many on this site.
- Most of my experience with XLR and various flavors of "balanced" connections are for pro audio, including live sound and studio recording. Made a big difference there, and some home environments have similar issues, though rarely as long of runs as live sound systems.