Nice pic Paul.
That [censored] has been going on for years. In most cases the amps on stage are for show and you'll get one or two that might be mic'd up and actually used. Word has it that the mighty Hetfield had two 1x12 Mesa combo's (not much bigger than a nice sized computer monitor) mic'd up under the stage in an enclosed room when touring in support of Metallica's Black album back in the 90's - even infront of ~500,000 in Russia... two little combo amps supplying the mighty rhythm tone that crushes all. Apparently the clean tone comes from a small Roland amp.
First time I spotted this shenanigans was in the early 80's watching MSG and the stage hands carrying complete stacks of two 4x12's off in one go apart from the stack with the mics in front of it. That was a WTF moment. Some guys like Yngwie, Dimebag and the guys from Slayer had all their amps hooked up and soundcheck just prior to the main act coming on always sounded ace if you were down the front.
Back in the 90's when I was also an aspiring musician I had a fun afternoon in one of the big 'sound' rental places in Manchester - Concert Systems. Their deal was mainly with renting big systems to established touring acts - they were sorting out The Prodigy's 120,000 watt system that was used in places as small at 4,000 seats. The guy there hooked me up with a Marshall SE100 speaker emulator. I'd had a chat with him on the phone prior to coming down to the store and he told me to bring my guitar and Marshall amp head. He hooked the amp, via the speak outs, to the speaker emulator and straight into a 2,000 watt small JBL system and it sounded exactly like a Marshall but [censored] loud - without hauling those 4x12 cabs. Even when he stuck a bass track and drums through the little mixing desk and had me play it still sounded farking ace. I still have the SE100 but the old Marshall was sold a few years ago for a massive profit 🙂 Bought for 100quid, sold for $1600. That amp was repaired at the Marshall factory in England, lots of 'new bits', including new valves, for 75 of the Queens finest and got me a factory tour too! Sweet.