Here are some pics of my new setup:
The main amplifier is an Audio Research Reference 150 (on the floor next to the left-side speaker). The subwoofer amplifier is a 500W Hsu Research amp (on the floor immediately next to the right-side speaker). I run the Sopra 3's full range without any crossover in the signal path. They start to roll off around 33-34 Hz in my room (measured with a signal generator and sound level meter). The subwoofers, a pair of Hsu TN1220's, cross in at 28 Hz with an electronic crossover internal to the subwoofer amp and measured response is flat to 18-19 Hz.