Who are some of elite sub woofers?


Looking to replace Klipsch 12d

So many choices.
Ag insider logo xs@2xawooof
REL is my favorite, hands down, after going through several lesser brands over the years.   I'll never buy another brand...if anything, I'd upgrade within the REL lines of product.   
Rythmik, HSU, and PowerSoundAudio.

SVS if you are limited to BestBuy or want financing.

REL and Vandersteen don’t even come close to Rythmik and HSU in terms of bang for buck. JL Audio also isn’t as good for the money.

As for ported or sealed. Ported gives you louder output in the deep bass but cuts off below what it’s rated at, sealed gives you less output in the deep bass but extends deeper (meaning at 20Hz a ported may be louder but at 15Hz a sealed may be louder), transparent companies will post frequency graphs (all 4 I mentioned do).

Ported subs can be “looser” if they aren’t designed great (<$500 ones for instance, but that’s a budget constriction), high end ported subs are no less musical than ported, and they have less distortion usually as well.  
  
If you don’t have DSP/room correction nor tons of bass traps, I would go with sealed, as the room gain would help make that more neutral than ported speakers.
If you are building your own, try a pair of Hartley 24" woofers in custom cabs that you can find designs for all over...

http://www.hartleyloudspeakers.com/new_page_2.htm

file:///C:/Users/Richard/Downloads/hartley%20box%20(1).pdf

You need a big amp (>200 RMS) and a big-boy crossover...

Awoof, one possibility since you already have a sub, is to add a few more.  In a distributed multi-sub setup, the subs need not be identical.   It is perfectly fine to have one or two that go significantly deeper than the others.  

If you go this route, let me suggest that any subs which will be closer to the listener than the main speakers have a steep lowpass filter, something like 24 dB per octave, to roll off their top ends.  You don't want them passing audible upper bass/lower midrange energy and giving away their locations. 

Also, a phase knob or switch can be nice to have.   I usually end up reversing the polarity of one of the four subs in a Swarm setup, as I find that this usually helps to further smooth things out.

Duke

Duke, I understand the subs in a swarm design all reproduce a combined left plus right (monaural) signal, even on material containing stereo bass (rare, but not unheard of). Is that correct? Is there any out-of-phase (left minus right), very low frequency, info lost when doing so?