I use a pair of Vandersteen 2WQ subs with Thiel CS2.4 mains. I don't know about other subs, but the 2WQs are tunable (volume and Q) and they mate very well with the 2.4s.
The effect in my system was not subtle. Yes, the low frequency material was there with good authority, but the sound all the way up through the midrange was cleaned up and tightened up; better timbral accuracy. Also, which was a surprise to me, the overall imaging and soundstage were much improved. These are qualities that Thiels are known for on their own anyway.
With the 2 subs its a whole new class of speaker system. Like I said, my impressions are solely in reference to the 2WQs.
In regard to placement, I think just put them where they fit best and tune them to that position. Mine are a couple feet behind my mains.