Sorry but you're wrong. Just like with sound, certain tastes can "mask" others, like how if you add sugar to a spicy dish, it takes away the spiciness.
Anyway, that's not important. The point is that there are complex interactions that happen when you mix things that can provide a more interesting/better "flavor" (be it taste or sound or something else entirely), depending on what you are mixing.
As far as subtractive/additive, that really makes almost no difference. They're all just "changes". Depending on where you start from and where you move to, the changes could be good or bad, just like anything else. Like if you have a bass amp that's deadly clean and you want some more grind, a little more gain would be a good thing. But if you had an amp that somehow only went moderate OD to sounding like a Mesa triple rec, adding more gain would probably be detrimental. Everything is relative. If you take a bassier cab and a more trebly cab and put them together, there's a good chance they will balance each other out. If you have 2 bassy cabs or 2 trebly cabs, they'll reinforce those more prominent elements and it will be that much more bassy or trebly, which usually isn't ideal.
Sure, you can search the world with a pocket full of $$$ for the single perfect cab and then get 2 of them, or you can widen your search and just find 2 cabs that might not be amazing on their own, but work very well together (for probably cheaper than a single "do it all" cab).