Are Expensive Balanced Interconnects Necessary?

The most common theory – which works well for me, because it coincides well with what I usually hear – is that single-ended connection (and gear) produces a natural spectrum of musical harmonics, same as in real life, while balanced gear (and connection) greatly reduces even (pleasant for our hearing harmonics), but leaves the odd (subjectively unpleasant, un-musical) harmonics virtually unmodified, unchanged, and thus DISTORTS the natural spectrum of harmonics, i.e. the ratio between odd and even harmonics, compare to life music.

@styleman ^that^ theory is total new to me.
The only thing I have ever heard referenced to balanced configuration, is the CMRR advantage of noise being rejected.

Never anything about harmonics. How would the cable even know what to add?
It is solely about minimising noise.

That's not at all how a differentially balanced circuit works. A differentially balanced circuit uses both an inverting and non-inverting signal at its input, then amplifies the voltage difference between the two signals to produce the output.

Depending on the design of the balanced circuit, there can be substantial benefits to using it in balanced mode, rather than SE. For example, with ARC balanced preamps, you'll get 6 dB more level using it in balanced mode. That means 6 dB improved s/n.

Maybe when noise is zero… @cleeds ?

But the idea of CMRR is usually more than 6dB.
Let’s look at each side:

• (+) S + N
• (-) -S + N

(+) - (-) = S + N - (-S +N) == 2S 

So, I gather that the answer to the original question is definitely maybe, or certainly could be.

@vonhelmholtz I think that it would be difficult to tell the difference between a $5 and $5000 XLR cable with any certainty in an ABX test.
But if it has been done, then we should be able to find it.

There are countless personal testimonials and poetic prose in magazines, that proclaim the money is well spent… just nothing that most would consider evidence being extant.

Clearly, single ended interconnects yield sound benefits as you move up a manufacturers product line, but do balanced cables yield the same improvement?

And not all the way up… sometimes different systems like different RCA cables.

The fact that recording studios are using regular cables, sort of suggests that:

1. It doesn’t matter 
2. or what ever was lost happened already

The best way to solve this, IMO, is to do what I did, which is buy the same exact cables, one set with balanced XLR and the other set with RCA (WBT RCA in my case) and test them back and forth in your system in your room with your ears

Nice answer @l1975r however the question that the OP asked was about expensive versus cheaper XLRs.

It is dumb enough to suggest that the OP buy expensive XLRs, when they will not likely hear any difference, but adding in an expensive set of RCAs which the OP does not need, is certainly going a bit far (IMO).

@marco1 - I appreciate your subtlety.🤗