now that its been established how Power and Xmax affect ported and sealed enclosures, how do they affect bandpass boxes?
For every type of enclosure type, output at the low end of the box is dictated by bore and stroke. (aka cone area and xmax)
There's only two real exceptions to this rule:
1) If you can restrict the radiation, efficiency will start to go up. This is how horns and waveguides work. In a car, the cabin itself does this, because the wavelengths exceed the dimensions of the car itself. This is why SPL cars invest a lot of effort to keep the car itself from flexing.
2) Horns and ports stop the cone from moving at certain frequencies. For instance, in a ported box tuned to 30hz, the cone is not moving. So voice coil thermal limits become a big factor at that frequency.