Anyone designed, built or using one of these in either their car or home?

Seems like a good way to get very good bass extension down to whatever frequency is desired.

I guess they are rather complicated to setup but once done what are the downsides?

what are they?
you got a url handy?

Just a kind of equaliser for subs.

Basically they are active filters designed to compensate for variations in a speakers frequency response so that the frequency response of the combined system is flat.

Yeah, its best to think of them like any other type of equilization..

Advantages

- very small box volume requirements.

Disadvantages

- Less efficient than a larger box that "naturally" has a lower Qtc and F3.

- Requires more linear excursion to reach a low F3 without distortion (although that will be no different to a sealed box system that plays that low without equilization)

Also, very few car subs should really need any equalization like this, but these filters can be very useful for a home sub.

Links -

http://www.linkwitzlab.com/filters.htm
http://sound.westhost.com/linkwitz-transform.htm