

Southfield High School, Detroit, Michigan, 22 Nov. 1967, with two Sunn 100S amplifiers and 2×15 cabinets, playing a Gibson ES-335. (Photo: Bob Elliott)
(Photo: Bob Elliott)
Beginning in August 1967 and for the early- to mid-1968 North American tours, Pete (and John) used Sunn amplifiers and cabinets, succeeding the use of U.S. Thomas Organ (Vox) Super Beatles, with Sunn 100S amplifiers and cabinets. The first known use of Sunn amps is 23 Aug. 1967 in Flint, Michigan. Its possible these Sunn 100S amps were purchased on 15 or 16 Aug. 1967, during a two-day break in Nashville.
The Who were first exposed to Sunn amps through the Blues Magoos roadie on the July-September 1967 Hermans Hermits tour, their first of North America.
The Who purchased additional Sunn gear, along with Sunn PA gear, from Mannys in New York, likely on or around the 25 Nov., 1967, Village Theatre dates in New York. Bobby Pridden had indicated that it was bought a couple days prior to the 29 Nov., 1967, date in Scotch Plains, New Jersey, though footage from the 25 Nov., 1967, Village Theatre gig shows Sunn PA in use. (Pete also purchased Coral Hornet guitars at this time.)
As a part of their use, the Who also provided endorsement for the Sunn gear.
From 8 to 14 July 1968, Pete tried Sunns new Orion solid state offering.
The Sunn gear was abandoned at Canadian customs in Toronto on 15 July 1968, at which point Pete began using the Sound City gear previously used only in the UK/Europe.
https://thewho.net/whotabs/gear/pa/pa6768.html