Fastest subwoofer?

Vandy 2wq is great and probably super fast since it has implemented Feed-Forward Error Correction, but may not be powerful enough depending on the room. If mono sub's not enough, I'd get 2x2wq that could be purchased for less than $500 used

My friend has Tannoy speakers that were difficult to find subwoofers that could blend with it signature. He finally came across a sub from Zu Audio and couldnt be happier. I heard it an its fantastic, if you have a chance try them. Not sure I champion Zu speakers but their subs rock.

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Not sure I understand.

If a subwoofer were fast, wouldn't it be a tweeter? :)
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Yes Hornguys, you can definitely "tweet it"!-)

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?