Best Subwoofer To Use With A Two Way Speaker System

I received my new Wilson-Benesch Vertex stand mounted loudspeakers in mid-January.  After about a 200 hour break-in period and a few position tweaks I am now in audio nirvana but without the final low octave or so.  I carefully chose my associated electronics and couldn't be more pleased.  My Sutherland N1 preamplifier has 2 outputs so I can use it to drive a crossover/subwoofer system.  
I am entirely certain that my spouse would never stand for me to add a W-B Torus Infrasonic Generator & the accompanying Torus Amplifier at about $12,000. I would like some assistance from those that have the experience to advise me of something very good to mate with my system at a much more reasonable cost.  Thanks

I've been using (off and on) a B&W ASW-650 for 15+ years with no complaints. If I were buying new, I'd look at a JL Fathom 12 or 13 (version 2) or a Velodyne DD-12. Either of these would more than meet my humble needs...
It would help to know what your other equipment is in order to determine what would be a good fit.
But, lacking that info, I would heartily recommend a pair of Vandersteen 2w, or 2wq subs and crossovers. They are by far the best integrating subs I have owned.
My system is:
W-B Vertex Loud Speakers
EKSC Eagle 11' Mono Blocks
Sutherland N1 Preamplifier
Custom shielded interconnects
VPI HW 19 TT w/ Ortofon TA 110 Tonearm & Ortofon Cadenza Blue
   Phono Cartridge
Custom W7 Silent PC with an Delta M-Audio Audiophile 2496 sound card     running WinAmp for MP3's and other software for higher resolution  
Kimber Bifocal X Loudspeaker Cables
Resolution/Imagining/etc. are nothing short of stunning.  Just missing the lowest octaves.
My room is on the smaller side however I have a very high ceiling and currently very little room interaction. I am willing to do some acoustic treatments and have some space to experiment with placement.
Perhaps check with W-B if there are any sub brands they recommend? Surely they realize not everyone who owns a pair of their speakers can afford a $12K sub. They may have some good advice and given the relative rarity of W-B in the marketplace might be a better source of advice. Also if you bought the speakers new how about the dealer?
Best Subwoofer To Use With A Two Way Speaker System
If you want to get HT type bass go with the multitude of 12"-15" units.

If you want one s/h that will blend perfect and give you an octave or two lower like it’s an extension of your Wilson-Benesch, then I suggest you look for an oldie but very nice Yamaha YST-160 or 305. These have proper linear amps twin 8" with servo control, very tight and musical, no doof doof droning HT sound from these. And they are the perfect height to use as stands, if you get two.

Cheers George