Musical subwoofer under $2000

+1 for the Rythmik F12G.  That's what I paired my Magnepan MMGs with (very, very well I might add). 

I later added a small M&K K-10 in an opposing corner just for kicks to help fill out the room.

The Rythmik subs are highly adjustable, appear to be about as fast as any sub on the market and IMO offer all the slam anyone could need regardless of music.  I don't understand the comment on Rythmik subs not being good for rock slam.  They'll eat pipe organ lows for lunch and drink rock slam as an aperitif, near as I can tell. 

YMMV.

I am critical of subwoofers and most recently  owned two REL T9i units. 

The RELs sounded excellent but had a couple flaws that made me sell them. The level control was too coarse for properly matching with my main speakers- one click position and the sub overpowered the main speakers and on the lower click the output dropped too low for some reason.  

They did not extend at the sub bass frequencies which is something I prefer- a strong output at 20Hz. 

My new subs are the SVS SB 2000 Pro.  They have excellent output and 20 Hz and give that deep bass sound and feel.  The App control is very fine- level setting and matching is easy.  The DSP is also useful, I used it to tame a few high peaks on the left subwoofer to smooth the response. 

If you have room, the SVS SB3000 is super impressive and has a bit better tone than the SB 2000 Pro.  The SB 3000s were too large for my space.  

I would highly recommend going the two sub route.  You have a decently large space to fill that will surely have peaks and nodes.  With subs 1+1=3!  You have a great system and some good subs will really enhance your listening experience.  Good luck and cheers. 

If you get two subs, you can also try each as L/R mono ( = stereo subs), and you can conversely stack them vertically and run each in stereo (L + R channel signals) to see if an “array” approach improves their “collective” directivity. Stacking may be more difficult with the model (Gallo TR3-D) I recommended vs. the others, but the benefits might (MIGHT) remain the same 😉