This is the best-rated sub on Newegg.
http://www.newegg.com/Product/Product.aspx?Item=N82E16882290130
What's wrong with it ? The amplifier board dies after
a year or two. I think you are familiar with this subject.
A friend at work, built his own sub. Or rather, a pair of subs.
We told him that human hearing perception cannot localize a
stereo sub signal, so you only need to build one sub. But he built
two of them, for "stereo".
The speakers were 36" in diameter.
The cabinets take up most of the living room wall.
The cabinets are filled with "ballast" to keep the
speaker on the floor. So you place the speakers,
then install the ballast. You have to remove the
ballast before moving the cabinet.
When they're turned up, the walls of the townhouse
begin to rattle just a tiny bit.
One advantage of the design, is the amplifier is
in a separate box. So the amplifier is not
subjected to heat and vibration from inside
the cabinet.
Now, that's a sub... This is an example of a 36" speaker.
http://autosom.net/noticias/20151203000114ds18_09.jpg
Paul