... They just have very low bass output under anything below about 70 Hz (to my ears) though they are rated for +/- 3db at 45Hz. Interestingly enough, my old SP 2/3s, which were smaller and rated down to 60Hz, clearly had significantly more extension connected to the same front end. I have moved them around to no avail.
What is the distance between the listening position and the rear wall? And have you tried changing that distance, in addition to changing the position of the speakers?
While the amplifier figures to be part of the problem, if the distance between the listening position and the rear wall is in the vicinity of roughly 5 or 6 feet, reflections from that wall can result in a large amount of attenuation of frequencies in the vicinity of 50 or 60 Hz or thereabouts. Perhaps that wasn't as noticeable with the smaller speakers because their different radiation pattern may have resulted in sidewall reflections that could have partially negated or compensated for that attenuation.
Divide the distance between the listening position and the rear wall, measured in feet, into 281.5 to derive the center frequency of that cancellation effect, based on the speed of sound in dry air at 68 degrees F at sea level, which is 1126 feet/second (281.5 = 1126/4). As a rough approximation I believe that significant attenuation may occur over something like a 2/3 octave range centered about that frequency.