If you want to control this at your fingertips you do! If not, you could always go with an external electronic crossover to do it for you. Coustic used to make some great budget electronic crossovers and Audiocontrol makes some great pricey electronic crossoversbut you must have a good HU with crossover to do so? (sorry to thread jack OP)
Actually it would be the human ear's perception of relative volume at higher frequencies. IE, the human ear is more sensitive to higher frequencies, which is why they sound louder when compared to the rest of the spectrum.x2 tweeters do not need a lot of power because of their efficiency