if you had, lets say, a 150W head and a 400W cab, what would be your total wattage? would it be 150W, 400W, or somewhere in the middle?
it is just a guess, but wouldn't it be the total power output of the amp?? The speakers being 400w would just mean that you can't over power them. I am probably wrong, it is just a guess.
What impedance is the head rated at and what impedance is the cab? MORE INFORMATION IS NEEDED
400w for the cab just means capacity it can handle from whatever head is driving it.