Help finding a Sub w/ quick punch down to 20hz

I am looking for a sub that goes down to 20 hz that is good for music and movies. My speakers go down to 42 hz. Any recommendations? Heard that the B&W DB1 and Definitive Tech Supercube 6000 are good, but not sure if they go down to 20hz?
JL Fathom F212, and the F113 as well. They are very fast and also go to 20 Hz. If you can settle for 21 Hz, you can save some money with the F112 ($3100 instead of $3900).
JL Audio's will do the trick as well as Velodyne DD15, DD18. I also can recommend a Revel B15(I have one) which is very fast and punchy and digs to around 18Hz but may be difficult to find on the used market. When they do pop up, used prices are around $1100.00 to $1400.00.

If you want real 20Hz response at low distortion you're going to need a fair-sized sub. Your room size in total volume is a big factor. To have a worthwhile discussion we'd need to know that, along with your budget. For example, the larger JL Audio subs (113, 212, 213) are nice, as are the Velodyne DD15+ and 18+, but these are all rather pricey. If you have the money, cool, but without a budget we can't tell.

There are several internet direct brands that are interesting, and a lot of people swear by. For example, the Funk Audio 18.0 and 18.3. The 18.0 is comparable to the Velodyne DD18+, but lacks some features, some design integration, although it has nicer cabinetry and finishes. The 18.0 and 18.3 are also available with neodymium drivers, which means they weigh a lot less, and can be moved around by one person.

Lower in the budget range is the SVS PB13 Ultra, which has prodigious 20Hz output in its ported mode. You might also look at Rythmik and the Hsu. The drawbacks to these subs are that they are somewhat less good looking, being simple boxes, and they have less powerful and sophisticated amps, but they get the job done at far lower prices than JL and Velodyne. All have low distortion and good time-domain performance, so they're fine for music.

Lots of people claim you get better results from multiple subs than just one sub, due to mitigating the effects of room modes. My advice is to start with one and see if it works well, like it did for me, but don't be surprised if two subs work a lot better in some rooms.

More than main speakers, subs work best when they have a lot of headroom, meaning you're not stressing them much at all. If you want decent sub-30Hz bass forget subs with a single 12" or smaller driver. A smaller driver and a big ported cabinet, like the SVS PB13 Ultra, can get remarkable output, but then you've got to put up with a huge box in your listening room.

I have a Velodyne DD18+, and it's a remarkable device, but lately Velodyne has been taking pricing lessons from JL Audio. Funk Audio has just released an improved version of the 18.3, and I'd probably go that way if I were in the market today. If you really want 20Hz bass and have $2K to spend the SVS is the way to go. For about $1K the Hsu is remarkable value.

I think all of the subs from mainstream high-end speaker brands are over-priced for what you get. Especially B&W. Revel is little better, IMO.

09-02-12: Irvrobinson
If you want real 20Hz response at low distortion you're going to need a fair-sized sub.
The REL F110 is about 13"x15"x17" and is good to at least 25 Hz. If you have a strong enough magnet, tight enough tolerances, and enough internal power, you can get to near 20 Hz *and* have serious volume in a relatively small package.

HomeTheaterReview's review of the F110 revealed that a single one could flap the reviewer's pantsleg at 10 ft.
The Svs ultras are really nice.
I believe all the Velodyne Digital Drive Plus models are rated well below 20 Hz +/- 3dB.

I replaced a single Digital Drive 18 with my third generation of Velodyne, two Digital Drive 12 Plus in my two channel system. It's a largish room with a vaulted ceiling. The two smaller subs added to the sound stage as well as the rooms bass nodes have lessened.
Check out Seaton Sound. The website, like so many ID manufacturers, leaves a lot to be desired. However, contact him directly and he will get you the information you need for your room and your budget. His subs are highly regarded by many.
Surreal Sound's Constellation sub-woofers will do it. You can custom select the number of drivers based on the volume of your room. "Dipole bass with an attitude" is their tagline, and it's absolutely true. See:

09-02-12: Johnnyb53
The REL F110 is about 13"x15"x17" and is good to at least 25 Hz.
Oops, I meant the JL Fathom F110, but you guys probably figured that
out by now.
Some really good sealed units have been mentioned already. The JLs are nice if your budget allows. I have a pair of the HSU ULS-15s in a large, open floorplan. They offer plenty of low end in my hard to tame room. I am employing the use of an SVS AS-EQ1 to take care of some peaks and valleys. I went to the HSUs from a pair of SVS SB12s. While the SB12s didn't have the headroom I needed, they did sound great and integrated well with my mains. Their new SB13 looks nice and should get you the Quick punch. I'm just not sure it would get you into the 20s.