Marshall amps are like wrestling with a beast.
Eddie didn't have attenuators around so he tried to lower the voltage on his Plexi because they were too loud. After destroying several amps using a light dimmer switch he got a Variac and that's how the foundation for the Brown sound was achieved incidentally to his lowering volume. Then you need the EvH chops which is really a lot of his tone also. Give someone else his rig and it will sound like any other Plexi.
That studio is playing those amps fully opened up and isolated in another room. The results through the monitors sound incredible. Can you imagine how loud it would be in the room? A guitar just sitting on a stand would be playing by itself with the resonance going on.