I've (briefly) paired my VSMs with a pair of 12" Rythmik subwoofers.
When I use subs, I use room EQ on the bass signal to the subs and run the mains unprocessed, using only an NHT X-2 active x-over to low cut that signal. I generally use an x-over point between 70hz and 80hz, because that's the lowest x-over point that still allows the room EQ to do its thing. This set-up has always produced both great bass performance and seamless integration with the main speakers of my choice (I've used +/- a half dozen, including planars, which are notoriously tough to integrate well).
I love the EQ'd Rythmiks - they dramatically improve the sound of just about any speaker I've mated them to. I say "just about" because the Merlin is an exception. I got the usual seamless match and improved power in the lowest registers, but the final result wasn't better than the stand-alone, for me. The character of the speaker changed and that spooky "U R There" character of the VSM was diminished. As I lowered the x-over point (10z at a time), the Merlins regained more of their native character, but the improvements wrought by the subs diminished. In the end, a "before and after" comparison (without subs before, with subs after) of the system struck me as a "less than zero sum" game.
Not sure why, but the VSMs are one of the very few speakers I've owned over the last 5 years that don't take well to subwoofers.
I'm sure that others would prefer the "after" in this "before and after" comparison - so YMMV - but I still run my VSMs by themselves.
Marty