They used Vox amps during their early US shows
Because of the expense and difficulty to ship and set up the Marshall Stacks.
It's kind of a misnomer, as they didn't use the Marshall Stacks for very long. The truth is that Marshall was one of the first to develop the 100 Watt Amp, mostly because John Entwhistle wanted to be loud enough to be heard over Keith Moon's drumming, which led to Pete wanting to be loud enough to be heard over The Ox and Moon.
It was an arms race after that, but if you saw them when in their prime, you couldn't hear straight for a week after the show.
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