Visible in the bottom right corner of this August 11, 1987 photo from Red Rocks. According to this historical article by Janet Furman (founder of Furman Sound) and this Dozin.com page, Garcia used the MC-2300 live starting in the early 70s.

Excerpt from "History of Furman Sound and its Grateful Dead Roots"

Not every Dead concert was recorded to 16 track. I only traveled with them for those that were, or occasionally I came along as the equipment tech. One memorable tour in July, 1973 included a stop at the Watkins Glen Summer Jam, held at the famed auto raceway. This all day outdoor concert featured just three super groups – the Grateful Dead, the Band, and the Allman Brothers. But that bill was enough to attract the largest crowd in rock history, if not in all of American history – 600,000, easily topping Woodstock’s smaller but better remembered crowd of 300,000. The Dead would need a massive amount of amplification to reach all those people. At the time, they insisted on using only McIntosh 2300 power amps, an audiophile rather than pro audio product, made in small quantity and hard to find on short notice. The McIntosh factory happened to be near Watkins Glen, in Binghamton in upstate New York. We were already backstage at the concert, and every road in the area was clogged with concert traffic. My assignment was to get five more of those giant amps, any way I could. Sam Cutler, the former Rolling Stones road manager now working for the Grateful Dead, handed me $6000 in cash and the use of a helicopter and pilot. Though it was a weekend and the McIntosh factory was closed, I tracked down the owner at his home. The pilot flew me from the venue to downtown Binghamton. Helicopter landings there were not an everyday affair, and there was great media interest. Flashbulbs popped and reporters stuck microphones in my face. In the summer heat, I was wearing only shorts and a concert T-shirt, with the cash wadded up in my pocket. I met up with the owner, who drove me to the factory and sold me the amps off the production floor. We drove back to town in his station wagon, his wife and kids aboard on their way to a summer vacation, and transferred the amps into the copter. At over 100 lbs each plus two people, it was a heavy load for a small helicopter. We had a very scary moment as we took off, coming within inches of crashing into a highrise building. But back at Watkins Glen, the sight of that enormous crowd from the air was unforgettable. In the moment I landed, delivering the goods, I became an instant hero.

Dozin.com

03/23/75. Mesa/Boogie MkI as pre-amp > MC2300 > 3 2x12's

(...) 5/77. 1 (silverface)Twin Reverb as pre-amp > McIntosh MC2300 power amp > 3 JBL 2x12's; BF Twin as reserve backup head

(...) Feb. to Mar. '80 JGB tour Mesa Boogie MkIIa head> MC2300> Hard Trucker JBL 3x12

Used during DJ Kool Herc's early career, as recounted in this January 31, 2018 Red Bull Music Academy interview.

Every time I would play out somewhere and come back from one of my parties, I would come back with a piece of the guy’s equipment that I’d bought.

What equipment?

I’m rolling with the big Mac [amp]. That cost like, say $1,600. A 2300 Mac, the biggest there is, the top of the line. The guy had top-of-the-line stuff. He had GLI, and the new company came out, he had the disco fours, and he had not one McIntosh 2300, he had two of them. And he had two [Altec] “Voice of the Theatres” [speaker system].

Who’s this guy you’re buying it all off?

He used to call himself the Amazing Bert. This system sounded like a band. People used to come just to hear the sound – they didn’t give a fuck what he was playing. What was coming through, it was crisp, you was hearing it. You could be on the Cross Bronx and be hearing this shit. But see, he was at Fordham University, he was a student from the Bahamas, so he had grant money. I didn’t have no grant money. So, he was getting grant money to buy all this new shit. I bought two of the fours, two of the other Voice of the Theatres, and I had one Mac.