Found this in one of the interviews with Nick Hipa about this album specifically:
"Amp wise, we got our rhythm tones with a Peavey 6505+ and that head actually beat out almost every other head you can think of. We tried all sorts of different heads for the rhythms such as a Randall head, a Diezel, a Engl, some Kranks, Mesa-Boogies, we tried them all, but the Peavey won out. It was the biggest shootout ever. And for the leads and stuff, we used a Soldano SLO-100."
I guess this is the answer... or at least I hope he's talking about the final reamped sound of the guitars...