Looking for Sub to pair with Dynaudio Confidence 20s

how about 2-REL t/7x or 2-REL t/9x subs? For 2 channel systems these are some of the best sounding subs in their price range.

REL is famous for easy implementation with a wide range of speakers using the high level inputs. REL has a 60 day home trial with free shipping. I owned REL from the early 2000’s with Totem speakers

@dmeltz 

Hi, As mentioned previously you do not want to do one subwoofer, two is the minimum. 

You have smaller bookshelf speakers hopefully on stands. These speakers have the ability to perform acceptably in a room that large , but it is pushing it. In order to get satisfactory headroom you are going to have to remove 100 Hz on down from them and hand it to two subwoofers at least 12" in diameter (the drivers) via a two way crossover. The best are digital such as the MiniDSP or dBx Driverack. 

If you are handy the least expensive way to get started in subwoofers is to build them yourself from kits such as those sold by Parts Express. They even have a selection of excellent plate amplifiers although I prefer outboard amps for subwoofers. QSC makes excellent amps for subwoofers at very reasonable prices. You can get two subs, amps and crossover for less than you could buy a single high quality 12" subwoofer from JL Audio. 

If you go with one subwoofer using a low pass filter you will be very disappointed in a 400 square foot room. 

Very simple

Get the most expensive REL you can afford. 
Simply tune to blend with your mains 

Good luck Willy -T

As soix said, you need to be looking at 2 subs without question. I would drill down on both REL and Rythmik's websites. Solid choices from top to bottom. I have a pair of Rythmik G22's and absolutely love em. They're a dual-opposed 12" model. They also make a dual 15" (G25). They're known for being smooth and easy to listen to, and I can back that up. The balls-to-the-wall home theater junkies think they don't have enough tactile response, and maybe they don't for that application, but that's exactly what drew me to them. The last thing I wanted was subs that draw attention to themselves. The goal is for it to sound like your speakers are producing all that bass.

REL is good but overpriced and lacks variable phase.