Sub question for my system

I run Thiel cs2.4's from a Parasound Halo A21. While the bass is very accurate sounding it doest have great low freq extension. I have an older M&K V125 subwoofer that has served me well in my HT setup and I'm thinking of trying it in with the Thiels in the 2 ch setup. I'll be using preouts from my preamp (both L and R channels) to the sub. My speakers are rated down th 37 hz or so, so where should I start with the crossover point on the sub? My concern is the M&K wont be musical enough to work with the Thiels, but im hoping that if its crossed over low enough then I might be able to get away with it. Failing that, and having to buy a more musical SW, and having a budget of $2k what are the better units to look at these days? Vandersteen is always good, but I'm open to suggestions. My room is 16x25 with 10 ft ceilings