Good talking to you earlier. Glad you went ahead and started your own thread.
I gotta be honest man. I got the impression that the db force is strong with you. Between your quest for SPL and your neighbor willing to hook you up, I have two (2) recommendations:
1. Offer to help him with his remodel. You will most likely be more of a nusance than help, but if he is willing to let you assist with the remodel, there is a very good chance that you could pick his brain on all things wood working. Trust me the knowledge you gain from an apprenticeship like that is worth more than a few free boxes!
2. Go with the G-Horn! With the UXL group buy going on at the moment, I don't see a better deal out there from a sheer output perspective.
Assuming you go the G-Horn route, let's piece together your system really quick.
1. UXL 18 x 1 - $410 Shipped
2. 4x8 Birch Ply x 6(?) - $300ish
3. MiniDSP 2X4 - $149 Shipped
4. IPR2-7500 - $700ish
5. Misc Finishing Matetials (speaker wire, speakon connectors, box terminal plate, etc...) - $50ish
There you go!
You are probably wondering why I recommended getting such a monster amp. First, it's been tested to be highly efficient. Second, it will future proof you because we both know that you are going to want to either to build another G-Horn or possibly an Othorn to compliment your G-Horn down the road.
Some people may chime in and mention the ringing issue that the G-Horn exhibits at 60Hz. If that turns out to be an issue, just save up for a Othhorn build down the road to let it handle the 50Hz up content.
Still on the fence about the G-Horn? Here is some context. 1 G-Horn = 4 Sealed LMS-U's between 16-32Hz (IIRC).