I was cruising around his (JS) website, and I was looking at his amp vids. He told a story of a famous English rock band's guitar player who was sponsored by Marshall. The guitarist had a setup of many Marshall "cabs" and "heads." But he said when he went backstage, they were all just empty boxes! He was actually playing out of a GIBSON 25 watt tube amp hooked up to the normal monitor and speaker system.

Personally, i think it is a great story.

But, Does anyone have any idea of what band this was?

or, Does anyone know of any similar stories?
dunno about a specific guy but i think its standard practice for a lot of big bands to use a wall of "fake" marshall cabs with only maybe 2 of them actually having any speakers inside.

although i do recall hearing about some bands using completely different amps to the marshalls that were visible on stage
A tons of pros do that. It's so they can make endorsement money. ZZ Top does that right now as do Satch and most of the Peavey sponsored shredders.
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