Plus, who says adding a second cab is simply about volume? Lots of people, including myself, mix cab types to blend their tonal colors. 2 4x10s is going to sound different than a 4x10 + 1x15, or a 4x10 + 1x18, or 4x10 + a 2x12... you get the idea. Then there is the fact that running a second cab stacked on top often gets the extra cab closer to ear level, and easier to hear.

