Can anyone tell me when XLR connections were first used on stereo equipment. (what years)?

RCA or XLR?

Should be single ended (shared ground) or balanced ground.

Single ended exposes the signal to RF (etc.) contamination.

@mewsickbuff Atma-Sphere was the first home stereo manufacturer to regularly offer balanced line products. The MA-1 amplifier was first in 1987 and the MP-1 preamp followed in 1989. Both not only had balanced inputs and outputs (including LOMC phono section) but were also fully differential and balanced internally. Both also supported AES48, the balanced line standard. The MP-1 also supports +4dB operation which is common with studio equipment no intended for home use.

The MP-1 also uses a direct-coupled method of supporting the balanced standard, for which we were awarded a patent.

Thank you all for your input. XLR has quite an interesting history. Can anyone recommend other stereo brands that used XLR in and out connections? Mid-fi  and no tubes is preferred. Thanks.

BAT, Balanced Audio Technology, is a great brand that makes all balanced equipment as the name implies.  They are built like a tank and make a great integrated.

FWIW, one of the founders of BAT was a customer of ours prior to the founding of BAT.