According to the UK Marketing Manager S (for "sound" was used as a reference label in the development stage for this series of keyboards, part of the prime objective being to concentrate on sound quality. When the product was lauched the "S" stuck.
Pretty boring really.

There's a podcast including an interview with the above + 1 or 2 demos here:
http://www.yamahapkowner.com/?p=206

[Don't have to be a member to receive it]

John

[This message has been edited by jwyvern (edited 08-10-2007).]