Fastest subwoofer?

What speakers are you getting the subwoofer for, and what's your budget? If you want fast and a good blend, the recommendations so far are helpful. I'm especially predisposed to the JL for anyone who can afford them. I've heard their Fathom F212's mated with Magnepan 20.1s and it was a seamless match. Anything that mates well with panels is probably a good bet.

Also, given your curiosity about the Paradigm Millenia sub, what sort of space considerations do you have?

01-09-12: Hornguys

Not sure I understand.

If a subwoofer were fast, wouldn't it be a tweeter? :)

No, because in addition to high frequency extension, a tweeter needs wide dispersion. The shorter the wavelength, the smaller the diaphragm must be to avoid "beaming." The woof/midrange unit in the GoldenEar Triton series has a frequency response out to 20KHz. This speed has often been cited as a reason that these midrange cone drivers blend so well with the folded ribbon tweeter. By the same token, Magnepan's end table-style woofer responds up to at least 7 KHz. It blends seamlessly with Magnepan's new desktop mini-panels. Both of these are drivers cross into tweeter territory but are still not tweeters. Still, the speed they have means they accelerate as quickly as the mids and tweets they are trying to blend with.

As to the subject at hand, REL and Sonus Faber are both distributed in the US by Sumiko, and there have been some packages put together by the two companies, such as the SF Toy/REL home theater setup. You could almost certainly get a good blend with a small REL sub. However, for a complete range of dynamics, power, blend, and speed, I think JL set the new standard.