I don't mean to rain on your parade (I really don't mean this negatively and please don't take offense if you're build is going to be this big and I totally missed it in the original post), but a single SAZ-3000D will pull a solid 300 amps of current at 12 volts. At 16 volts it will pull 500 amps, which is the limit of 1 awg wire. Two SAZ-3000D's means you better be prepared with the capability to feed them.
But to answer your question - 1 12 w/3k watts = X. 2 12's w/3k watts = X+3dB. 2 12's w/6kwatts = X+6 dB.
...in a perfect world anyway. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif