Sounds like this ones gonna be loud.

I am not the best person to get into this, but in case noboby brings this up I want to mention the "array" issue. With 15" drivers I think you will have to cross over below 800 Hz. The difference in distance between the two drivers off axis can do nasty things at higher frequencies due to interference. So if you had them alligned vertically the sound of the mids would get nasty if you stand up (or if you sit close). However, if your design permits the low crossover you will get more juice from your amp from a 4 ohm load, and some increase in sensitivity.

I think that your observation about reduced excursion is astute, but I can tell you in my setup there just isn't any meaningful excursion at any volume most people will allow. If you need high output though, more drivers is better than overpowering fewer drivers. You'll get less power compression and less distortion.

You may want to search the archives here for posts on 2.5 way systems. I find them fascinating.(2 woofs, 1 crossed at say 1.6 KHz, and one crossed say 500 Hz. If you use woofs with louder midrange or are going to put them far away from walls, this works well.)

Thomas