Using ESL speakers I needed to make sure any sub purchase would keep up with the speed of the speakers. I recently ordered a pair of Rythmik servo F12Gs. Couldn't be happier.
I cross them over at the bottom range of my speakers and I now have seamless integration that reaches into the teens.
The subs arrived in less than a week after my order. Two days of fiddling to get them right, and I now have exactly what I hoped for, bass without limits, no boom when it isn't called for, but full support when it is. Symphonic classics are now a joy to behold. Once you get concert hall realism nailed, jazz and rock fall right into place.
Regardless of what you pick, I do recommend getting two if you can. They disappear this way.