WOT,
Please see the graph. It is the Xmax plot of an IXL-15 in a 3 cube ported box with 1000 watts input. You will notice that the only time it reaches Xmax is at about 21Hz. At that frequency, there is very little, if any music content. Music begins to disappear below 30Hz. Under normal conditions, folks apply a filter - usually at around 25Hz - to ensure minimal driver damage at very low frequencies.
So, you can see that Xmax as a measured parameter means little here. The 1000 watt power handling refers to the thermal power handling of the driver (at least in the case of the IXL). It does not have anything to do with excursion limits.
XBL^2 - this is a design that attempts to level off (or maintain) the BL factor when a driver's excursion increases. It's claim to fame is * lower distortion at higher excursions. Since you will be applying 1000 watts, you will not be taking full advantage of XBL^2 as you will not be achieving high excursion with the power you have.
Now, you want volume. This can be achieved by increasing the tuning frequency of your box. The result will be increased output, but with a slight "peaky" sound quality. One term a person used recently to describe this sound was "violent".
So, if you want your system to slam, I suggest 3.5 to 4 cubes per driver tuned to 36Hz.
* - I use the term "perceived" as I have not seen any empirical evidence showing an actual decrease in distortion values (e.g. distortion curves) of said technology.
