Multisubs don't help smoothness across the frequency range per se, they allow for reduced spatial variability, a smoother response over a larger area. Without EQ correct positioning or other adjustments more sources can make the situation worse not better.The W371 is way out of my price range regardless!
Considering my problematic area is basically everything below 200Hz, I suppose the ideal setup for me would be the D&D 8c with properly set up multi-subs to get smoothness across that whole range? I can't help feeling I'd miss out on the benefits of the coaxial Ones as my position on the sofa is quite variable.
Right now, I'm crossing over a single sub at 120Hz to improve my sub-bass response (speakers can do down to 40Hz normally) but this creates a wide dip between 100-200Hz . I've identified one more possible location to test the second sub although it suffers from low WAF!
Instead of buying new speakers why not start a thread, post some information and measurements and get some help to make what you already have better?