Who are some of elite sub woofers?


Looking to replace Klipsch 12d

So many choices.
awooof
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..  
Within reason, I have a very healthy budget. I almost fell into the "pay more it must be better" trap. I didn't and bought the SVS's (Ultra SB16) and kept the rest of the money. I am very happy with the pair of SB16's I purchased.

The day you let your ears and not your wallet do the listening is the day you become a man my son.