Which subwoofer?

Take a look at HSU research subs.  They are of fantastic performance for the money. 

Thank you all for your thoughts. 
I’m really liking the idea of the RELs. There are multiple dealers in my area that will allow me to get a listen. 

I’m not one to mess with two subs, especially in a smaller room. Bass, below 100-120 Hz appears non-directional. I did not say non-directional dogmatically. The mid-bass and midrange give the sense of directionality, so two subs for this purpose are not required. And with placement, integration, crossover, volume and phase, one is quite lovely. 
The key is proper room pressurization. 
I have a Rel T9/I and a medium room - exquisite.
look up the new T9 and check the extension very nice and very low for its size.  

I`m using 2  12" HGS Velodynes with my 3.6 Maggies the integration is outrageous, I also suggest looking into a integrated amp or tube preamp & SS Power amp combination in the near future.

I find myself in agreement with @jwillox . I moved from a pair of ML Descent i sub’s sporting the optional crossover cards with my ML CLX Anniversaries to a more centrally positioned single ML Balance Force 212 and preferred the ease with which a single source base extension integrated with the main speakers and room.

That said for 80% of my music predictions I switched the subs/s off as I prefered the more natural midrange of the CLX’s with their own integrated ESL base panels which measured down to a solid 46Hz -4db  in my room.