Subwoofer recommendation for Dynaudio Confidence C4

2 subwoofers offer better room and system integration. You get better seat-to-seat consistency, less peaks and dips, less localizability, deeper extension or louder output, etc.  
  
REL sub’s are a bit over-rated.  
  
If you don’t have bass mangement (dedicated subwoofer out), and this either need to use REL’s Speakon method or XLR passthru, look at something like a pair of Neumann KH 805’s or a pair of Rythmik E15’s with the H600XLR2 amp option (if you want a nice looking subwoofer, Salk will build you the kit version of the E15 in whatever finish you want, plus they make the cabinet thicker and improve the internal bracing, so it also performs better, just specify you want the H600XLR2 amp, this will of course have a wait time as they have to make it).

@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.