adding a sub to a tower is kinda tough...it is not a efficent design.
the more a sub is reinforced with another plane - the louder it gets.....( adding a surface close to or around the sub adds 3db... 3db is liken to doubling your amp output...)
that is why speaker sound like crap when they are not mounted to anything..subs in most(99%)installations need reinforcement
so when you enclose them and place them by/on the floor(+3db) and then place them toward a corner(2 walls - add +6db) and have a close ceiling (another +3 db)....you just picked up 12 db.....which is HUGE !!!!!!
that means a sub(on the floor, by the corner, with a close celing-400 watt amp) will sound just as lound as a sub with the 1600 watt amp out in the open. and in this case louder...
bass waves are very, very long. ( take 550 and divide it by the frequency and that will give you the length.
40 htz note will have a 14' length,
30htz note will have a 18' lenght,
20 htz note will have 27.5 ' length !!!!!
(that is why most car systems sound louder with lower frequencies outside of the car)
generally speaking, since low bass is non directional ( must have the crossver point below 80hz) you can pretty much out the sub anywhere. so a custom isoberic with 2 10's requires very little space(hovever alot of juice jl class d amps work great) and will actually sound and feel better since it will use the long sound waves can travel througout/inside the boat.
here are the drawbacks.....
the subs will rattle anything that in the boat that is not mechanically grounded or secured
the mid/higs will have to be delayed 3-5 milliseconds ( the mids and high will reach your ears much faster than than a bass wave..so they need to be "delayed" till the sub wave can make it out and not "cancel" out around the crossover frequency ( most car systems suffer from a lack of midbass in this freqency range)
the high eclipse and alpine decks will allow for the altering the phase and/or delaying the phase so the all the freqencies make it to you at the same time.
hope that helps,