20 Hz Alpine horn sub

This is a custom horn sub designed for 20 Hz extension. It is designed to be installed, hence the dual opposed mounting to avoid vibration transfer to the rest of the house. 

First comment. Hornresp makes things look worse than reality. The peaks don't turn out quite so bad, and the vertical scale makes them look worse. For these two reasons, measured results will usually tend to look better. This one could be used to about 60 Hz. Subs such as these aren't ideal where you might need to cross higher. The first peak around 90 Hz must be avoided. I think these are best to use in a system where you have something else running the upper bass. In this system there are dedicated active woofers.

Here you can see that the design loads the cone down to 20 Hz and below that a high pass filter is required or the output will be severly limited. If one were you use a sealed box to avoid the need for this, the amount of air the drivers must move is increased by a factor of four.

In terms of getting the maximum output for the least total spend, this kind of design can be a good value for money option. In terms of sound quality, it will depend on implementation more than anything.