In my comments, I essentially suggested the same thing. Amplifier output impedance interacts with the rest of the components in the output circuit. Not only does it act as a voltage divider but reactive properties in the system are modified. The electro-mechanical properties of the motor/drive interface are shifted, so the speaker acts like it has higher Q. Also, the behavior of the crossover and any other electrical components are shifted.

One thing that you couldn't help but notice was that the sound from the speaker was pretty strong in the bottom octave when connected to a Paramour amplifier. That led me to speculate that the Paramours are somewhere close to being in that condition where BSC values equal zero. It sounded much like what you'd expect from the same speaker and cabinet driven by an amp with low output impedance and BSC with 6-12 ohms series resistance and maybe a couple millihenries inductance. That's not very scientific 'cause there were no measurements, but you could hear about 6dB more below 100Hz with the Paramours. Certainly enough to be noticeable.