Subwoofers

Gee, an $8K  sub for a $1K speaker.
I think the OP might want something a little less expensive, even if they are worth every bit of it.

Thanks, gillatgh.
Hey, OP, have you come to any conclusions?

One would assume that the perfect subwoofer pairing would be separately powered subwoofers built-in to the main speaker or what one may consider as a truly full range speaker. 

My experience with this type speaker systems low frequency extension (no experience with Vandersteen) is limited adjustability and most importantly limited placement. Modern subwoofer control technology can make possible placement, as it relates to room node reduction, more relevant than subjective sonic pairing. Using long inexpensive line level cabling and mounting subwoofer/s on dollies to find that rooms best subwoofer location/s  can greatly reducing room nodes without sacrificing the sub/s gain and maintaining optimum equalization.
  
https://www.audiogon.com/listings/subwoofers-velodyne-dd-12plus-black-2016-05-17-home-theater-91201-...
With one or two DD Plus (used 10"s for less cost) one would have the flexibility to attain extremely accurate and flexible integration. You'd also have the ability to model the presentation of most any other subwoofer you desire and store it in one of six custom EQ'd presets as well as other remote controlled adjustments such as volume and phase which can be made from the listening position. Phase often changes from track to track on a single album.

Go to Velodyne.com and compare their Digital Drive Plus User's and Interface Manuals pdf's to most any other setup and placement information supplied by other manufactures. 

I'm using a REL T-9 with my Magnepan 3.7i's.  It integrates quite well  but the supplementation needed in my room with the 3.7i's is rather minimal.

LarryRS