Charles, sure that would be of some concern, but don't overlook the key point that Arthur has only a 100Hz crossover available on his receiver. If the sub is reasonably near the mains a 100Hz crossover, rolling off the sub at 24dB/octave, isn't likely going to be a significant problem. The STF-2 that I suggested certainly is a fine performer in the 100Hz area.
Any sub that I'd be interested in would certainly perform very well at frequencies much higher than 45Hz, and I'd expect it to play louder and with lower distortion in the 40-80Hz octave than the main speakers.
