Subwoofer recommendation for Dynaudio Confidence C4

karurravi,

     It sounds to me like your system is for music only, no ht duties.

     If so, you're probably only concerned with good bass response at your listening position and don't require good bass response throughout your entire room.

     The Audio Kinesis Swarm 4-sub system, that I linked an Absolute Sound review of, is what is called a distributed bass array that provides sota bass response throughout the entire room in virtually any room provided the progressive locating procedure for each of the 4 subs is followed. 
     This system utilizes a 10" long-throw woofer in each of its rather compact cabinets(2 x2 x 1 foot).  It's ideal for music, since each sub is providing only a portion of the total bass, it's capable of both taut and tight bass and deep and powerful bass that is felt as well as heard.  This also allows the system  to possess large reserves of power for impressive bass dynamics. 
     It is true that 2 subs will provide better bass response than 1 sub and 3 will perform better than 2.  However, 4 subs properly positioned has been scientifically proven to provide the best in-room bass response.  This results because 4 sub distributed bass arrays reduce or eliminate the vast majority of bass standing waves in virtually any room which, in turn, reduces or eliminates the vast majority of bass room modes (spots in the room of bass exaggeration,  attenuation or cancellation) throughout the entire room.
     So, it's basically your choice; is very good bass response at your listening position sufficient or do you prefer very good bass response throughout your entire room?  Cost is really not an issue since the 4 sub AK Swarm system (including 1,000 watt sub amp) is priced about the same or less than 2 very high quality subs ($3K).

Tim

@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.