MoD,

RE "LE 15 spider has (I think) less stiffer spider"

True, but it applies to spider and surround. The LE15A Vas parameter of 736 L represents a large volume of air and as such it is more compliant (the opposite of stiffness), compared to a smaller Vas number leading to a stiffer suspension.

So that data gives an indication about a woofer's suspension (more or less flexible), plus it is also used to determine the proper box volume for a speaker, along with the driver Qts.

Richard