Looking for Sub to pair with Dynaudio Confidence 20s

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.

Rogersound Labs was the first company I heard that had a subwoofer back in 1974.

I have a pair of their 10S Speedwoofers and they pair really nice with my towers. I gave the 10S Speedwoofers a try first due to having transmission line design to go lower in a musical fashion.

The 12S Speedwoofer has come out and it is very highly thought of. They are a company that will let you try them out and pay the freight to return [at least they did when I was delivered my pair of 10S]. I didn't send them back.