Good question about what studio amps Entwistle used on "Who's Next."
He was using SÛNN earlier than that and also later. I am thinking that he mainly used SÛNN for his live North American tours and Hiwatt for the UK/Euro, due to the cost of transporting it all ?
But I know he used SÛNN for the Magic Bus album.....the doomed Orion Series of SÛNN, which was the first SÛNN attempt at solid state amps. Those were designed with the large heads as just the preamps and the power amps were in the speaker cabs.
What was happening was, since The Who played so damn loud even while,recording, the power amp got really hot and then failed due to the vibrations from being inside the speaker cabs. SÛNN had the ship more over to complete the recordings.
Shortly afterwards, SÛNN CANCELLED THE Orion Series and recalled any that that been shipped to dealers. That is why you VERY rarely see any. The grill cloth was a very distinctive silver with long black vertical stripes. I had one Orion combo amp and one of the large preamp heads....but those were the only Orion survivors that I found in some 20 years of collecting SÛNN.
I spotted one of the 2x15 cabs in an old live shot of Berry Oakley with the Allmans....and there was an old SÛNN ad of the guitarist of Blue Cheer in front of a huge wall of those cabs...but that is about it before they killed off the line.
But Entwistle did use them for, I believe, the entire Magic Bus album.....between using the earlier SÛNN 200S tube heads and then all the later solid state Coliseum models.
I DID have a prototype COLISEUM MODEL T that had been Leslie West's, of which there was never an official.model. It was built on the larger 1000S/2000S chassis and had a similar layout to the later Model T. You could see various rivet holes in the chassis where they had made changes and moved various components as they tried variations. But that is also evidence that SÛNN liked the "Coliseum" name before there was a solid state line.
I also have seen a couple of very early Model T production heads marked on the back as "Coliseum Model T."