Just ten more dB made that much difference? SB does make low distortion drivers, but only at low volumes..... 1.2 kHz is too low for tweeter to play, so I don't think move crossover will work. From hificompass' measurement the 1 kHz longer decay shifts towards 1.2 kHz, don't know if they originate from the same thing. 1 kHz looks like resonance , 1.2 kHz looks like surround issue. It is such a weird driver to work with. If you want I think you can try edge coating the driver to stiffen up the surround to cone area. It is documented some where on the internet, one example is Trovel G....'s website. I don't know the consequence though, I guess the Kms vs excursion curve will be more ugly, but given the fs of the driver is low I would expect three way crossover at a few hundred Hz would experience no bad effect.