well i've personally built 4.5 @ 29, 4.5 @ 32, 5.5 @ 32, and 5.9 @ 39.i haven't had a system for awhile or built a box for like over a year, so yea lol... i did some quick research on here and people said 5cubes for 2, so thats what i did... you sure they can take another 1200?
5.5 @ 32 was loudest....i was gonna lengthen the ports on the 5.9 and make it 32 because i think it would have been even louder.
Don't be scared to go big...I'd recommend 5.5-6 cubes for 2 if you want pure ground pounding lows and to rattle your jaws loose. it still sounded good as well.
I had my 2 on a Hifonics Hercules....the same amp was benched at 2400 watts at 1 ohm. After rise, etc I'm guessing they were getting around 2200ish total, 1100 a piece. I never had problems with them and I wanged them pretty hard. If you're gonna put that much power on them, you HAVE to be smart and set the gains right. If you don't know what you're doing, just keep 1200 watts on them.