Emerson got one of the first Modular Moog synthesizers, if not the first, and that's when things really started to take off for both of them. Moog had been making synths since 1964, but they were mostly seen as "experimental" instruments. Moog gave that Modular to Keith around the same time as the first ELP album, and Keith figured out how to do things with it that Moog hadn't even imagined. ELP helped put the Moog, and synthesizers in general, out there in the public ear.
Keith stuck with primarily Moogs for years, only switching to the Yamaha GX-1 around the Works sessions. It was very different, and I think just doing something different and creating sounds in a different way was attractive to Keith at the time.
You are right, I remember photos showing him with someting looking like an old telephone switch board, plugging cables in and out. It was really fantastic what he got out of it, I guess he spent 100s of hours learning how to operate it.
More on The Nice:
Although Emerson used most of the time on the Hammond, he used to switch to an upright piano and perform endless traditional jazz improvisations with Davidson. I didn't fancy it too much, neither did the audience I think. It was still a rock /pop audience and traditional jazz was close to a no-go at these concerts at the time. I think he left it, when ELP was formed, even though they had a few songs of this piano-jazz style on their repertoire.
It was sad news for me, when he decided to leave The Nice. I think he had two reasons to do so:
Jackson couldn't sing. He sounded like a frog.
Neither Jackson or Davidson had the confidence /charisma to fill up huge stadions - but Emerson had, and he wanted it.
So he got Lake and Palmer, and by this hit the right spot. As seen with famous bands, the right combinations suddenly clicks and they create unforgettable music.
In 1970, I was at the Fehmarn festival in Germany and heard ELP for the first time. I think it was 3am at night and I barely remember what they played. To be honest, we smoked a lot of weed and drank many beers, for three days, in a terrible weather. I don't know how I survived this. At the same event, I also witnessed the last performance Hendrix did, before he died shortly afterwards.