Four 8s beat three 10s...but in a boat?

I have an x9 and am considering replacing the rear the rear, under the seat sack with 500# of lead and four 8s. Originally i was thinking of Solobarics (s8l7) but now have heard great stuff about JL Audio 8w3's.

1) are four 8s worth adding?

2) how will four 8s compare to the S12L7 i have in the bow? would a 12 and four 8s be DAMN loud, or just loud?

3) what 8s are best considering small box volume, less than a cube? No IDMAXs or 8w7s...

Consider that all four 8s have to be powered by one old school Fosgate pushing about 1400 real watts. Thanks for comments and advice...

Currently I have two Resonant Engineering 8's ported under the dash of our boat. Have two channels of a us acoustics 4085 bridged for about 240 watts. Most people would be pretty impressed. Well outdoes a single ported 12. 8's are really a good option, you'd probably be impressed. two only need around 1 cube tuned to 32-35 hz. you might want to look into RE as well. Fairly inexpensive and a great woofer. I have one RE hc in my tahoe off 1400 watts. scored a 145.7 on a linear x in an sq box. So they definitely get loud as still sound great.

Is no one else using 8s? Four 8s has to be as loud as one more 12 right?

4 8"'s will be louder then 1 12". Simple math, way more surface area with 4 8"'s.