Easy way to test if MAB affects damage on a magical attack:
Double weaken yourself (die again while already weakened) and use an attack. If it consistently lands for 0 damage, MAB is a factor. As far as I am aware, there are no exceptions to this rule, but I wouldn't be closed to the possibility of someone bringing an example of one to my attention.
Double weakness sets MAB value (which is a multiplier) to zero, so no matter what sort of numbers you're getting from other damage modifiers, they will get multiplied by this zero, which will make your damage 0 no matter what. This happens even if you use a Primeval Brew, so people using magic attacks to brew with have to be careful not to let themselves become double weakened before using the brew.
Edited, Apr 27th 2011 6:03pm by Fynlar