goonstopher, very good answer, the Submersive has been proven to be a very good performer and now even more so with the extra power. All the things you mentioned and the finish options are the reasons the Submersive is quite a bit more expensive than the 18.2 sub.
Also, don't forget that the DSP besides equing the low end, also controls the sub's behavior when it reaches its limits. In other words, no sub bottoming, weird noises, etc. That amp is very expensive and sophisticated to say the least.
Having said that, I do believe that the 18.2 is a great sub for the money. At its price point, I don't know if there's a commercial option out there that's better, maybe the ED kit of dual 13's w/LT1300 plate amp might be close. Those are heavy duty drivers with serious xmax and great SQ, even though they are on the small side. I believe it's a little cheaper than the 18.2, but you have to put the kit together, so it involves a little DIY.