Why did you decide on 4 cubes each? FI recommends 4-8 cubes. Is it for power handling ability or just size limitation?
I've been fantasizing about getting a Suburban and throwing a pair of the same FI Q 18's in the back, but going with the full 8 cubes each.
By my calculations just the driver displacement and bracing I/you will lose about 1 cubic foot per driver, so a 4 cube box must actually be about 5 cubic feet each. MY 8 cube box will have to be about 9 cubes each, unless you mount the drives inverted on the outside.
I too would like to hear them first, but not alot of people go for the big sealed SQ drivers that you and I want.
I'm thinking of a AQ2200 or even Sundown 3000 for power.