I've heard the GTI's, here's where I'm at in regards to your posting
In regards to box making, cant help you - outta my control on that I'm ignorant after an extent.
GTI is a great sub, will handle a solid 1200-1400 daily, but once again if you beat on them, then no telling how long they'll last. They get pretty loud, but they are SQ oriented.
9500 doenst sound horrible, but far from SQ in my eyes. Will it handle 3500 daily on a pair? For a while, yes, but how hard are you beating on these subs, honestly? Seems like pretty hard.
Instead of buying subs rated for 750-1000 rms and feeding them near double that, why dont you consider something that sounds good and is made for something like that?
RD Audio Alpha is what's been in my MazdaSpeed3 until I sold it, I love it. On 1000 watts it was loud, on 2000 it was stupid. Sounded great as well
Havn't heard the Flatline(although I'd love to) and the only IDMaxx I have heard was sealed, so that's not much. Clean sub though, I will say that
But honestly, consider something with stronger power handling. Its evident you want SQL and that limites your choices, severely, considering you want to send 1750ish per sub in a daily setup, most subs of that caliber are very SPL oriented.
HDC's will handle it all day - will they sound as good as you want? Probably not Same with the BTL's and the MT's
Somethin else to consider possibly is a BL? It's not rated for quite that much(going against everything I just said) but every comment I've read about it has been "feed her more power"
Anywho, hope it's somewhat helpful, best of luck