Ampeg SST 4x12 cabinet

[meexiko]

I think they used the cabs that way because the volume of the ampegs is massive. If I remember correctly Josh used a VT-40 as clean amp (ok, not so clean, but his idea of clean tone) and two heads, maybe two V-4, or maybe a V-2 into a V-4 that goes to the two cabs.
I remember a video where Nick moved the cabs to let the sound coming out more straight to him, like he needed to hear better what he was playing.