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

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

^ Steve Bednarsky?

@coltrane1 Yes. He had an in law that lived a block from me. Steve had two sets of our MA-1s and one of the first production MP-1 preamps. He was a friend of Victor, who called me one day saying the simulation he was running of the MA-1s didn't work. I pointed out to him that the actual amp obviously did so the issue was the simulation. It requires a schematic as an input, one which Victor made on his own. This was several years before BAT was founded. We still have Steve's warranty form for his MA-1 on file. 

@atmasphere Steve is a truly nice guy! He and I had many conversations, and all the while I was thinking, don’t you have more important things to be doing than talking to me? Nope, Steve always took time to speak to people, and for many years he was the one answering the telephone at BAT. 
Victor Khomenko was nothing short of a genius. Both of them built a wonderful company that still, some near 30 years later are building great gear. And I still have a VK5i preamp that still works. 

@coltrane1 Yes- they built a nice company.

They were several years after the next company we found after us to be making balanced line equipment; Jeff Rowland. IIRC their balanced preamp showed up about 1991.