Cannabis Rex will give a very big smooth foundation. They're also quite clear, they don't add much distortion on their own. IMO they work best in combination with some more aggressive speaker; put two Cannabis at the bottom of your 4x12" and two more aggressive speakers in the top position and you should be very happy.
Or, for the ultimate metal cab, use two Eminence Black Powders in the top position and combine with something that cuts the mix well like 2 V30s in the bottom. Then blast away.