My first choice would be DSS, I love the way my setup sounds!! A demo will leave the unseasoned person alittle off balance and head stuffy for a few hours, lol! With that out of the way the SP4 is a top notch sub,The FI Q is a terrific sub. I owned 6 FI subs and had good results from all of them. The SSA Zcon is a awesome sub and sounds like you have the room for one as they seem to like alittle larger enclosure (4-5cu for a 15), The Evil is another sub that is badass. SSA makes a great product. With the space you have available I would run 2 12in DSS Ethos on a 3k watt amp. That will out perform 1 15 and you wont have to push them as hard as a single 15 to get the same output.