Who are some of elite sub woofers?

I'm guessing this subwoofer is to be used in a two channel music system?

Multiple subs will defiantly load or pressurize and reduce or eliminate room nodes within a listening environment better than one. With proper placement and equalization you should find very enjoyable performance at your listening position.

Regarding your budget I suggest you look into Syzygy Acoustics.

http://syzygyacoustics.com/#

Unfortunately, their site doesn't seem to offer their manual for the possibility of slaving your current 12d from the remote controlled room corrected Syzygy or running two of their 8" models. I'm sure they'ed be happy to answer your questions. 

In my opinion room correction is far more valuable than giant driver size. 
Good luck with your search and have fun.

Wow, Duke in the house! This is pure gold!

Warning: The following three paragraphs go into some technical stuff. If you don’t like technical stuff, please skip them. 

A single equalized sub can give excellent results in the sweet spot, but often makes things worse outside the sweet spot. This is because the room-interaction peak-and-dip pattern is specific to that sub location and that listener location. Thus in other listening locations the peaks and dips will have moved around, such that in those locations the EQ could be boosting a peak and/or cutting a dip. 

With a distributed multisub system, the multiple dissimilar peak-and-dip patterns sum at any given listening position, and therefore tend to smooth one another out. The only way they could fail to smooth one another out would be if they were identical, and that won’t happen unless the subs are all in the exact same location. If there is still a significant residual peak or dip, chances are it’s present throughout the room, and therefore is a good candidate for correction via EQ. In other words, a distributed multisub system actually makes EQ more effective (though less likely to be needed), because its benefits are more likely to extend throughout the room.

And "smooth" bass is "fast" bass, because it is the in-room peaks which decay more slowly than the rest of the bass spectrum and therefore sound "slow". Yes we can hear the difference in perceptual "speed" between different types of subs, but that goes back to which is creating the biggest in-room peaks, and it will be the sub that is loudest in that region. 

Anyway imo you are WELL ahead of the game with three distributed, equalize-able bass sources. Earl Geddes, whose ideas I use (with his permission) in the Swarm, later moved to using three independently-equalized subs, with the equalization settings generated by his own proprietary algorithm from in-room measurements. I’m no Earl Geddes, so I’m still using his first-generation four-piece distributed multisub concept.

I wish I had sufficient economies of scale to do a $1500 budget-Swarm system... alas, not even close. The labor cost on the boxes doesn't go down significantly as the box size decreases, and labor is my biggest cost.  

Duke

Three paragraphs I wish could be pinned to the top of every subwoofer thread. This is like one of those puzzles, where everything seems impossible until you learn the trick, and suddenly its easy. Don't brute force it with one. Finesse it with four. 

High on my upgrade list for this year. Very high!

Maybe outside of your budget, but I personally love my dial JTR Captivator 2400s. They are port tuned to 10Hz, so you get the high output of ported, but with the benefits of sealed. They are very fast and sound great for both HT and 2 Channel music. Check them out.

http://jtrspeakers.com/captivator-2400.html

+1 for Rythmik.  Exceptional value, highly adjustable, very musical.  I love my FG12.  With its own adjustability, it integrates seamlessly with my Maggie MMGs through a Parasound P5 preamp. 

For music it is Rel, Rel, Rel.  They're not amazing home theater subs as they don't scare the heck out of you when something explodes on screen.  But they're awesome for music - they don't make the room shake, they make vocals sound real.  I also agree that more than one can even out the room response tremendously..