I always thought it was odd John went from Sunn to Marshall to Hiwatt, then back to Sunn again with the 'Coliseum Bass' solid state amp, and various cabs in the mid-70s. I love Sunn, but Hiwatt is next level, imo.
In the UK, they started with the customary Vox, then got in on the ground floor with Marshall as Pete's dad had played with Jim Marshall(on drums)in a big-band situation. In a volume war with John, Pete actually proposed the first Marshall "stack", although it was originally a single 8x12" which was soon cut in half.
When they finally started to get US airplay in 1967, and first toured here, they were not a big deal band with a budget, so they were provided American made Vox gear, solid state, very little to do with the tube/valve British stuff. They hated it, but when they came back for a second round, they got hooked up with original era tube Sunn equipment, far more beefy than the Vox, and akin to the Marshalls back home. But even that situation was changing.
Pete had a falling out with Jim Marshall, then he and John took up with Dave Reeves, who had created Sound City amps as a house brand for a London music store, but was developing his own new line based on SC with higher quality components. That of course was Hiwatt. John was still interested in Sunn, and now had money to spend on a new rig. Sunn's original owners had sold out to a corporation who proceeded to phase out tube amps, but did a great job developing a new transistor line with modern tone features and all sorts of speaker options. That then was the Coliseum line, where he went further with the multiple sized driver concept, a bass PA/wall of sound to which he later added pieces from a German brand, Stramp.