where he said they (him and Malcolm) each had SIX 100 watt marshall
Yeah Mr P that's right but what they have ONE main amp (a 50watt Marshall JMP with preamp, crank the shit out of it to pull the sound by driving the valves), and then feed into whatever many 100watt heads they need for sonicness/bigness, then feed the speaker boxes for wall of sound, then P.A it to make go far and wide (so you can hear it from your house).
Yepp you're dead right there. Just remembered an interview on "The Guitar Show" on the ABC where Angus said pretty much what you just described (one 50watt head to get the 'out front' sound, and the 100 watt heads for his on stage sound/volume).