The way I understand it is the mid range will get 150 watts in it's frequency range and the tweeter will get 150 watts in it's frequency range. The crossover is there to divide the power into the proper ranges. For example, you may have a passive crossover with a crossover point of 3500 hz. this means that the roll off point for both the mid and tweeter are at 3500 hz with some sort of slope. The mid will not get too high above the 3500 hz and the tweeter will not get too much below that. Again, this will depend on how steep of a slope the crossover has. Essentially, everything at and below 3500 hz will play on the mid with 150 watts of power. Everything at and above 3500 hz will play on the tweeter with 150 watts of power. Now, the tweeter may be receiving a little less power than the woofer if there is a tweeter attenuator on the crossover and you're using it. Also, some crossovers have resistors to absorb some of the power that would otherwise be going to the tweeter in order to protect it. Some also use little fuses that blow if the power gets to be too much for the tweeter. This is my understanding of how components use crossovers to divide the power. Anyone correct me if I'm incorrect.