Mike, xo and power handling will dictate max output. That is to say, with a lower xo, you'll have to limit max power (and therefore max output).
If you haven't, read through Tux's recent build with the Celestion woofers. He worked out a few different xo designs with different points and slopes, for different goals.
Slope is the missing element to your calculation at this point. The way slope and phase work together is going to have a lot to do with the elegance and natural sound of the result. And of course, slope will in turn have a lot to do with power handling. None of this operates in isolation.