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