Well i tried about 4.2 with an 8" sono with flares on it. Probably the loudest on the lowend so far but i switched it again today and have it at 3.75 with a slot and will listen to it on my drive to work tomorrow.
Didnt like the slot port as much as the sono so i took it back out. If i could make the box a little shorter and maybe cut off a fee inches of the rear deck i think it would work better. Sub fires up into the shell and sub is only 2.25" away from the top so i think its too close.
Turned my shell into a box for the tsns 15 so ill see how i like that tomorrow.
I was running two custom 4hp 15s in my car for a while. 3 cubes each, port was too small(like 11ish sqin percube) but it was amazing low end and sounded great.
***** about the top assembly. Have you thought about making some custom recones?
Ya id like to get another 5200 motor and build 2 15's out of those, but that would cost probably atleast $750 for another motor and 2 recones and baskets.
Im going to put the 18 back together probably next week when the kit gets here and maybe try to get rid of it for some 15's.
Ill post a pic of the kit i bought when i get on my laptop.