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 can't recommend more highly the B&W DB 1 sub that I've used for several years in a very high end system currently with Wilson speakers. You will never know it's in action except that your bass response will be awesome seemingly from only your main speakers. It also has great features such as room correction. They have recently updated these subs with remote control among other things. At $4500 list I can't imagine spending more and used ones when available(rare) fly out the door. Last thought. Get two if you want perfection plus they look good and are relatively compact.
You may find the best subwoofer is no subwoofer. Make sure you try before you buy. You dont want to muck up the speakers sound quality that you now enjoy... with an inferior sub.  
Thank you to all of the above.  A home audition is a must as "aolmrd1241" expressed.  I am thinking that it boils down to three of the brands that seem to do most things correctly, in no particular order: The B&W DB1, Velodyne & JL Audio.
Any other critical requirements?
If your amp meets the requirements, you should strongly include the Vandersteen 2Wq subwoofer. It is one of the best integrating subwoofers around. It's super fast and is great for a 2 channel system. IMHO.
Perazzi28, I find the Sutherland N1 preamplifier very interesting. Appearance wise I especially fancy the nixi tube numerators. If you have a moment I was wondering what you were using for a preamplifier previously and your impressions of the N1 especially the phono section.