As for your last question (subtone):
I am a professional saxophone player, and I consider the VSL sax-samples (I only have the ones from the Special Edition) as following:
There are 3 velocity layers in almost all patches, even in the legato (exept for bari, there are two), which is unusual for legato patches.
The lowest velocity is subtone, the second one is regular sound, and the loudest is espressivo. So when changing (or crossfading) from one velocity to another you completely change the way of playing. A solution could be to "scale" the velocity of just one vel-zone. If you like to use only the subtones, just don't go over vel55 and use either the expression cc to control the volume, or expand the dynamic range with a plug in.
But I second that the VSL saxophones aren't meant for jazz ...