My guess is it's like:
Pro Edition Version 2
An upgrade, with V-Span (I assume velocity spanning), 24-bit, more articulations, and an updated and extended performance tool set. And perhaps a modular format (ie: it's own smaple playback engine, or at least compatible with several big ones.).
Anyone who thinks its simply the Horizon and PE bundled at 24-bit with more tools, is likely wrong. They didn't put out Horizon to make it obsolete. And they didn't wait this long just to copy and paste samples into a bigger bundle. No, I think this is an extension/upgrade from teh Pro Edition, a sort of Pro Edition 2, obviously called the Cube.
Evan EVans