My ten favorite commercially available subs because I'm a biased man.
In no particular order:
JL Audio Fathom f113 it's small and sleak with incredible SPL in the upper ranges. It's also not ugly. I would probably own one if I didn't build my own sub. (I prefer the shorter one's looks)
Funky Waves 18.3 I love PR-s and I love TC sounds. They do a fantastic finishing job and you get the best bass possible
SVS Ultra Cylinder. I love the idea of having a couple tubes in my theater. It's innovative and effective.
Seaton Submersive. I love subs that look good. Sound good and deliver. This one certainly does.
HSU STF-2. It's a fantastic value at 349 that won't break a budget. They make other great subs, but they also offer one of the gems below 500.
Elemental Designs A-3 300. Another value sub offering at 600 they offer deep notes without breaking the bank.
Danley TH-Spud. Need a new step for your theater. This sub will amaze you while pulling less energy than your traditional subs. A tapped horn build is an innovative approach that uses a small driver for big sound.
Epik Empire. Another great sealed sub under 1k.
CraigSUB SS-18.1. Thought he's not a popular figure in many circles his strong opinions do lead to very solid subs and the passive option might be preferred by some. He offers a unique option.
Velodyne Optimum In-Wall. For those who only want in wall options Velodyne actually offer a decent(expensive) approach.
There are many great subs for many situations that's why it's always good to ask for help, but there are numerous options with great companies, great products and great engineers pushing the envelope. The idea that 2 or 3 companies are the only ones offering great subs points to a lack of research.
