SQ difference XLR vs RCA ?

Balance components are basically designed to have one side mirror the other side with respect to the electronics inside the chassis.  When manufactures do this they tend a cheap out because they have to do two systems.  So if you get a RCA only device there's no mirroring of the internals so the belief is that more money spent on the electronics you're buying.  Conrad Johnson is a perfect example, they only do RCA connections. 
 

Based on my experience XLR has always sounded better.  I can't think of a single time when I used XLR's that I wish I had used RCA.

Based on my experience XLR has always sounded better. I can’t think of a single time when I used XLR’s that I wish I had used RCA.

I am sure we could find some other piece of gear that would not sound better.

Balance components are basically designed to have one side mirror the other side with respect to the electronics inside the chassis. When manufactures do this they tend a cheap out because they have to do two systems. So if you get a RCA only device there’s no mirroring of the internals so the belief is that more money spent on the electronics you’re buying. Conrad Johnson is a perfect example, they only do RCA connections

What does Conrad Johnson have to do with anything?
50-75 years ago, no one was using XLR to my knowledge… Maybe NASA, and maybe Pro Audio were??

The use of XLR for pro audio brought better CMRR and lower noise.
It also brought higher signal levels, which helps too… as that also allows lower amplifier gain, which also helps from a noise perspective.

@russ69

Just to be an old curmudgeon, RCAs have worked just fine for me over the last 55 years.

hey russ -- you don’t know what you have been missing all this time... 🤣🤣

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@mijostyn

Balanced cables always sound better whether you can hear it or not.

so when an xlr interconnect gets unplugged in a deserted forest, does the sound degradation truly exist?  😂😅

Some DACs are a SE design. In these cases there would be little to no improvement using XLR unless a long run required.
More of an improvement if the DAC is a balanced design.