It's definitely positive feedback. If you put a dot of paint 99% of the way down a spring, that point moves 0.99 times as much as the end of the spring does. The superspring replaces the upper 99% of the spring by moving the mounting point of the bottom 1% in just the way the paint dot would move. Negative feedback will effectively shorten the spring.
The limits on the spring length are basically stability and SNR--if your loudspeaker isn't linear enough, it'll oscillate in high-gain regions and have degraded response in low-gain regions. 0.99 is about as high as you want to go, although you could probably do 0.999 if you linearized the speaker really carefully, maybe with its own sensor and feedback loop.
Really low frequency negative feedback (say another factor of 10 lower) will stabilize the operating position of the mass without doing much violence to the system response. Supersprings can have nasty turn-on transients, so you have to be a bit careful designing one, but it's a fun exercise. People have done it using a servomotor and a reel of cable instead of the speaker. You can get a much larger deflection range that way at the price of much more vibration.
Cheers,
Phil Hobbs