passive crossover related questions- calling all experts!

[nobody special]

I would like to design a speaker system, but I really don't have access to software such as leap or calsod. If I have the data supplied by parts express, such as impedance, phase, and SPL/frequency, would it be possible to simulate a crossover circuit with spice, and manually figure out the summation of the driver responses?
I see it is pretty easy to figure out what voltage changes will occur through a passive network, and then using simulation and calculation, I can see what this will do with the drivers, as far as voltage is concerned.
Some of the questions I have are about phase response, and calculating SPL.
Theoretically, speakers in phase will produce a 6 db gain if they are both fed equal power, right? Would this only be for a point source, or for speakers which are physically less than 1 wavelength from each other at that frequency? What happens when the phase angle is not 180 degrees out between drivers, but isn't in phase completely?
Is it possible to calculate the phase angle and voltage to the driver with the network, and manually figure out what the system SPL will be? Or, is this way too complicated to do without software? I don't mind working a little harder, if it is possible.

[PaulHilgeman]

Have you looked at speakerworksop over at www.speakerworkshop.com

???

[nobody special]

No, haven't seen that one. I will check it out. Thanks for the link!