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

Note that there is a difference between XLR connection and balanced operation.  Much of the benefit of XLR connectors arises from being used with a balanced circuit,.  However, as Ralph mentions above, there are plenty of ways of having balanced in/out connections without using balanced circuitry throughout the device.

If you're shopping mid-fi, there's an increased chance of running into devices using XLR into a not-truly-balanced circuit.  I'm guessing that all of the brands mentioned above are fully balanced, but defer to @atmasphere

PS I believe some Threshold preamps & amps had XLR connections in the 80s and 90s, but I don't think they ever made an integrated amp.  Much more recently, Parasound HINT & HINT 6 integrated amps include XLR ins & outs.  I don't know if any of them are truly balanced, but both companies were/are highly reputable.

Note that most consumer gear with XLRs only support 1 balanced input/output, as balanced consumer gear is rare.  Someone with a good knowledge of pro gear might be able to steer you to some other alternatives.

Accuphase 1980       

Accuphase is designed and built to last a hundred years 

XLRs were first used by Ampex on Microphone cables in the early 1950s. 

Ampex set the PIN ID as Pin 3 hot, Pin 2 cold, and Pin 1 as ground. 

In the late 1980s or early 1990s several manufacturers changed the PIN ID for Pins 2 and 3, making Pin 2 as hot and Pin 3 as cold. 

High end audio started using XLRs in the early to mid 1990s. In the early to mid 2000s or so, high end audio started offering fully differential circuits along with balanced connectors. There are still quite a few manufacturers who offer XLR connections on home audio gear but the signal is not fully differential.