I read an interview in one of the guitar mags with Angus a couple years ago. He stated that in the studio they used Marshal 2x12 combos. I seem to recall JMP45, but I could be wrong about the model. Definitely a combo, though.
The record does not sound like a 4x12, to me. It does not have that standing wave that I hear on a closed back cabinet. And it certainly does not sound like JCM800. The distortion is too natural and farty to be a JCM.
The amazing thing about these records is I always feel like I can hear the mahogany in Angus' guitar. If you've played enough old Gibson SGs and LP juniors and specials, as I have, you become very familiar with the resonance and overtone of these guitars. It's totally in the wood. It's so light and porous. As a finish carpenter, I've done my whole home in mahogany, and it really has such a completely different feel and resonance from the other woods typically used in guitar making. It also absorbs lacquer so much more deeply than other woods, because of its porousness.
One of my complaints with JCMs has always been that it does not matter if you're playing a Gibson or a Charvel, it's going to sound pretty much the same, because the overdrive of that amp is such a dominant characteristic. It just eats up so many of the natural overtones of the guitar. That to me is another big clue that this is not the JCM amp, because I can hear the mahogany.
Feel free to flame away at these assertions, but that's the only way to describe what I'm hearing.