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.