What would be better? I need opinions from all aspects; sound quality, loudness, bass output...etc...etc...

I will be running 1200-1400 watts regardless of which setup.

The box will be sealed.

 

If you have that much power RMS, i'd say go with the two 10s. i currently have one 10 in a ported box with a clarion 1000.1 amp on it. wired into 2 ohms, i'm putting 500 rms to the sub and its loud and sounds good. i can keep the gain all the way down and the sub level on my radio between 1/4 and 1/2 and it sounds great!
i think two 10s in sealed boxes splitting that power would sound even better. i am about to switch to a single 12 (two 10s isnt an option for me right now with my available power, i want some more room in the trunk) with 1000 rms on it.

 

Sound quality and L7 in the same sentence..... i would look elsewhere....

 

I'll agree somewhat with this sentence... I'm looking to replace the 2 12's that I've got right now... if I can get the $$ together I'm stating to consider the Adire Brahama's....