In many applications you may be very happy with the outcome. Just leave room for the Tweeter, so that you can update the design later. X-overs are not so difficult if you get the Behringer analog x-over.

The problem is that the waves cancel when one driver 1/2 wavelength farther from the ear than the other. At 10k hz this is .67", 5k would be 1.34, 2.5k would be 2.6". Also the sound will drop off faster at the higher Fq.

The applet called "the edge" is great for visualizing diffraction effects. You can see how 2 point sources create a null zone.

If you have a favorite spot that you like to sit in you could focus the array. This will place all the drivers the same distance from the ear.

-Linc