Jafant, as the 3.7's are missing the first octave; they are not full range. For that sub woofer augmentation is necessary. The 3.7's have their own different qualities, attributes. All of the Thiel CS models have the same placement recommendations. Many of Thiel's earlier models (pre-Thiel sub woofers?) had deeper bass, including all of the previous CS 3 series.
BTW, a 20 X 20 room would not be desirable.