Using sealed boxes. Would a single FiQ18 in 7 cubes or (2) FiQ15's in 2.6 cubes each hit the deepest?

Size of cone does not dictate frequency response. But if you have room for 7 cubes, and Fi suggests it, the design should be impressive. I usually prefer less drivers.

My question isnt which will get louder. I already know that (2) 15's will hit louder because of more cone area. But would getting a single 18 be any advantage over getting 2 15's. Soundwise. I dont want to hear "less wiring"......

well naturally the 18 will take half the power of the 2 15s to get going therefore less electrical strain, but sound wise alone i cant really see any benifets

you might meter a Hz or two lower if you really wanted to, could your ears hear the difference... I doubt it. songs play the same freq. indifferent to the subs its going to, so it will sound the same be it 15"s or 18", you just get a little more cone area out of a 18".

just FYI, my TC sound 3000 15"s can be tuned all the way down to 10Hz, so size doesn't matter...lol.