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
A pair of Vandersteen 2WQs will offer a much better in room response then any one sub.
It is the only sub I know that will not only extend your bass and play your music by lowering the distortion dramatically increasing clarity dynamics and transparency
This system offers a wide range of preferences with a nicely written support manual to get it right.
yes am a dealer... JohnnyR
Perhaps get a REL. You can use one of 'em with great results and I'm NOT a dealer, just a very satisfied owner of an old REL Q150e that is amazing. My listening room is a bit larger than yours and has a tall sloping ceiling...the single REL loads the room with vivid lownessness, adjusted to meet the lower area of my mains (Silverline Preludes) at about 45 hz or so. I adjust the level a little here and there for some stuff, but not too much.
Im liking the concept of the Vandy subs, but Im a bit confused. Is there a crossover you need to buy? Based on their diagram it looks like the leads come from the amp straigt into the amp of the Vandy? Is this correct? If so does this change the load on the amp? Im already running 4 ohm speakers. What's the max watt input to the Vany sub amp?