Shadorne, from my minor experience. . . JRDG 7M delivering max 170A peak ad damping factor of 170 is borderline for controlling bass drivers of Vienna Mahlers. At 1000, JRDG 501s and 312 seem just about right. . . Nuforce at 4000 is perhaps overdampening them.
My point is that you can't control them any further than having an amp with 0 output impedance (although you can go negative but that is another story).
Damping of 4 is equivalent to output impedance of 2 Ohms whilst 40 is equivalent to 0.2 ohms whilst 400 is equivalent to 0.02 whilst 4000 damping is like 0.002....so the difference is very small as all the last three are basically pretty much all a short. So the natural mechanical/electrical damping of the voice coil and mechanical suspension + air suspension will really begin to dominate as you go above 40 damping from the amp.
However, what you observed means it still has an audible effect which suggests the speaker design is probably underdamped.