Velodyne DD-18 vs Martin Logan BF 12”

I like subs. However, they can produce more room modes, and that sounds horrible. Therefore I would advice to go for multiple smaller subs (at least two and perhaps four, if you can find locations for them), plus room equalization like the DSpeaker Antimode 8033. See here for some introduction:
http://www.acousticfrontiers.com/20101029using-multiple-subwoofers-to-improve-bass-the-welti-devanti...
http://www.theabsolutesound.com/articles/dspeaker-anti-mode-8033-dsp-subwoofer-equalizer-tas-204/

Velodyne DD 18. It has some corection and perhaps your HT proc does too.

Would more subs be better? Depends. I like the idea of having one for the rear surrounds. One for the mains. Even though there is usually but one dedicated sub track.

Using an extra sub means extra stuff and blending it to which ever speakers. Lo pas. Hi pass. Cut offs. Etc.

Velodyne DDs can be daisy chained and give great results via the on board room correction.

Sorry, I can’t speak to the ML option. I’ve only had Velos.
Good luck.

+1 blindjim

Even the early DD-18 (not the Plus) Room Optimization is some of the most detailed on board equalization available. Coupled with its remote controlled preset flexibility it allows the user to make many on the fly adjustments. 

It bettered both my now very dated Onkyo and the Pioneer Elite receivers in dealing with the extra low frequencies in my room. On the other hand in my opinion it's just TV and using most modern receivers 80Hz sub output and the receivers on board EQ can do a very good job using an optimally placed subwoofer with basic controls.

The DD-18 is 123lbs. and the ML 212 is 140lbs. You might consider a dolly and very long interconnects to locate your rooms optimal single subwoofer placement.