Sean...A speaker (driver) is most like a PM DC motor. Except for static friction, (if any) which causes a "deadband" for plus/minus a small voltage, the torque constant (KT) of such a DC motor is completely constant over its useful range. We used a servo motor with more than 400 inch oz of torque capability to measure gimbal bearing friction of about two inch oz, and gimbal unbalance to an accuracy of better than a tenth of an inch oz. Perhaps you are thinking of AC motors, which do not develop much torque at low rpm. And, BTW, the DC motors in the PWM gimbal servos that I worked with "changed direction" (when commanded) at 300Hz...not exactly tweeters, but certainly comparable to woofers.
Apart from broadband sensitivity, frequency response might vary with SPL. I can do that test too!
Apart from broadband sensitivity, frequency response might vary with SPL. I can do that test too!