Dynaudio 18S

Its suprisingly compact and dense, and really goes down to 16hz at resonable monitoring levels!

However, the DSP (and AD/DA conversion) adds a total of 2,4ms of delay to your audio. So even if you don´t use any speaker/sub-distance in its settings you will always get a minimum of 2,4ms of latency in your audio. I have connected it on a separate out and had to delay my main monitors by the same amount (minus their own DSP-latency, Adam A7v which has a 1,7ms internal latency). This is if you can place the sub in the same plane as the monitors of course.

In terms of sound i´m really pleased with it! It is punchy and very precise, not boomy like most ported subs (this is a sealed design, giving you a smaller box-size at the cost of higher power consumption).

The settings of the sub are very advanced, i was able to almost notch out my lowest room mode at 33hz with the built in PEQ while using REW and a measurement mic resulting in a very flat room response. Same with the next room mode at 45hz. If i ever change to an analog sub, i will add a MiniDSP in between to do the exact same thing as this is more important than any subwoofer spec.

Placement and room acoustics is of highest importance since the room always dominates in the lower frequency ranges. Expect to spend some time tuning the integration!

A nice side-effect of the dual facing 9" elements is that the sub does not vibrate at all!

Overall, i´m very happy with it and thinking about adding a second one..

I give it 4 out of 5, mainly because of the 2,4ms latency and the lack of information of this in the documentation. Had to ask Dynaudio about it.

I have no complaints about the build and sound quality. I have lots of positive things to say about them, but honestly, I can't be bothered at this point.
It arrived with default DSP firmware, and while I was playing with that for a couple of days I upgraded to the newest one at that time (and this time, unfortunately). It did bring improvements/features, but some went missing or introduced bugs.
While going through the settings and testing I noticed the following:
- missing 40-120Hz LPF, only Dynaudio speaker presets available. I was using 80hz one cause they didn't even bother making a BM5 MKIII preset. New FW forced me to settle for one of the speakers. Regardless, integration was very good in my case, but I can see how this may pose a challenge for someone wanting to use very low or high frequency - there aren't any!
- changing sub's sensitivity would change the sensitivity of speakers. This was no more, very good.
- auto-standby would kick in no sooner than 1h I believe. Now it's down to 10min - very good
- introduced HPF, which was a Godsend as it turned out. I was fighting nasty peak at ~40hz and whatever I'd do in sub's PEQ, it wouldn't go away entirely. I realized afterward that speakers still had a decent output at 40hz, no matter if HPF at speakers was engaged or not. Switched to sub's higher-order HPF, which stopped speakers in their tracks, so there was no more interference. PEQ worked with no issues all along - very good. Not so good was distortion right around 80Hz with HPF ON for the first time. Did a factory reset/power on/off and never heard it again
- SUBstantial improvement of performance below 40Hz. Bass didn't roll off below 40ish as it did before. Improved SPL handling at those frequencies. Before I could barely reach 90dB in my room at the lowest frequencies before overload protection kicking in at those frequencies, now it's closer to 100dB. Higher the input from there, the higher the frequency being clipped. Still, it did about 115dB at 40Hz and up, below is rolled off/clipped. Probably could've gone higher, but... neighbors. So don't expect from this fairly small enclosure to rock your world with 110dB of 20Hz goodness - very good anyway.
- screen doesn't time out after 5 seconds of inactivity while being in the middle of menu options, trying something out - very good

General:

- PEQ allows you to bring the level down to the middle of the Earth, but doesn't allow you to raise it at all. While I understand and agree that huge dips shouldn't be messed with, it would be useful to have +6dB option at hand for some different situations and purposes.

So, am I dissatisfied because I encountered some of the FW issues and bugs?! No, what so ever.
I brought the attention to Dynaudio regarding LPFilters missing, and they responded promptly, saying that they 'disappeared' from the update and that there will be a new one by the end of March if everything goes according to plan - obviously hasn't. I understand the Corona outbreak has been nasty, but it doesn't seem to justify why a feature-packed, 1400€, midrange-high end flagship doesn't have a properly coded firmware after 3 years?!
I agreed to postpone my review until after I get a hold on the new FW, but this is getting too much. Might revise it after they release it eventually.

Edit #1: Mostly grammar. Support assured me the update is going through the final testing and should be out by the end of the month. The job was bigger than anticipated. Hopefully, I'll eat my own words.

Edit #2: Update has been finalized, YAY. Couldn't find anything wrong with it, it's mostly bug fixes.
Added bm5 preset. Auto standby isn't variable anymore, it's fixed to 20min now That's about it.
If you want to get most out of your DSP, I'd recommend taking a look at KH750 DSP. I certainly wish I had.

This is an amazing sub.

It's mind-boggling how something of this size can reach so low and do it with such speed and authority.

I've owned 12", 15", even 17" that cannot reach this low. Sure, a 15+ will move more air an, but for a studio? You won't need more power than this - and the speed and precision it has can hardly be matched.
It compares favorably with a 15" Servo motor sub (Rythmik).

The integrated DSP is the cherry on top. I use an external DIRAC processor (and thus leave all processing disabled) but the advantage of being able to integrate it the proper way without additional equipment is very welcome (yes, it's precofigured to make it easier with dynaudio monitors, but you can configure it crossover precisely by hand).

For whatever reason very few subwoofers have proper, selectable low pass AND high pass filters. The one integrated in the 18S has selectable slopes, which is a great bonus.

I was conflicted between one 18S and two 9S... I'm still not sure I made the right call, considering I'm not taking advantage of the DSP at all, but I can always add another one and the performance is so good that living with my decision isn't hard at all.

I've been using Dynaudio LYD 48s for a few months now with an old Blue Sky sub. The LYD 48s are capable of going down to 32 Hz, but they just don't carry much weight down there and the Blue Sky sub helped. I love the sound of the LYD 48s, but holy #$&! the 18s improved my system dramatically. If you can afford the 18s, go for it. You won't regret it. Living in the US, ordering it from Thomann saved me hundreds of dollars too. I asked Sweetwater to price match & they told me that I should order from Thomann because there's no way they could come close to that price. Happy camper here.