Tom,
4.0a (New sounds, new styles, new 4.0 OS).
4.0b (Option to have old/previous SD1 sounds with the new styles and new 4.0 OS)
4.0c (4.0b PLUS more polyphony and smaller issues resolved).

If 4.0a works well for you, keep it.

Thanks,

AJ
_________________________
[KETRON - USA]
Design Engineer & Product Specialist.
www.KetronAmerica.com