Subwoofer recommendation for Dynaudio Confidence C4

@audiotroy 

Rel makes some of the best woofers on the market known for speed, accuracy and integration.

I don’t disagree that they make well performing subs, just that there are equally performing subs for cheaper

Salk makes decent speakers using other peoples drivers. Rel or JL Audio design from scratch everything, this generally means a better performing product.

Disagree. It may mean their costs are lower as they don’t need a middle-man, but that doesn’t mean they make better sounding drivers than RAAL, SEAS, Scanspeak, etc. if the drivers are well performing, the crossover is well designed, the cabinet is inert, etc. who cares if the drivers are a third party, as long as the end result is good. There have been some “showdowns” between different equally priced speakers that included a Salk speaker, and everyone I found ended up saying the Salk sounded the best, including ones that were blind testing. I’m not saying they make the absolute best for the price hands down, but the certainly are up there.  

for music servo woofers can often times be considered slow. 

Untrue, at least for Rythmik (who Salk uses). Look up measurements, their subs usually have impulse response that is fast and clean, group delay is below 1 cycle, well controlled decay, etc. They are very tight and fast subs, even their ported models.

Look at a pair of 18.0 or 21.0 from Funk Audio. Nathan can match the finish of your C4. 

If you don’t care about finish that much, may want to look at JTR S1. 

Try a JL Audio e110 or e112. Great subs.

I took over a year to decide on my last speaker upgrade, the Dynaudio's were "finalists" in my search, eventually bought a then brand new model, a pair of Focal Sopra No2's.  IMO both these speakers' bass response is good, for as low as they go, but IMO there's no speaker in that price range that can truly compete with a great sub woofer in the low bass region.
I compared the JL Audio F112V2's with the F113V2's and chose the latter.  IMO, the F112V2's might have been a better match for a pair of stand mounts, as they go higher up in the bass range than their larger brother.  But with the bass that both the Dynaudio's and the Focal's produce, either model would contribute a lot to the final sound.
As for going with a pair of subs (that's what I did), there is something "visceral" in the musical experience when a low bass sound is coming predominantly from one channel or another.