You're mistaken - ARC was building balanced, differential amplifiers and preamplifiers long before the design became popular. The Reference One preamp goes back to the mid-90s, for example.
@cleeds Atma-Sphere was the first to offer a balanced phono input, back in 1989. The sales manager of ARC contacted me about how that connection was done (mijostyn and lewm did a good job of explaining it above) since he saw us demoing it in a store in LA back about 1992; I sent him a copy of our MP-1 owner's manual that explains the connection and a couple of years later ARC had a balanced input.
To my understanding they didn't keep it on their preamps, likely because dealers didn't like changing tonearm cables in order to audition the preamp in stores so they went back to single-ended. I was too dumb to sort that out and so maintained balanced inputs. After 30 years this sort of thing is a lot more common, but 30 years ago was a different world...
