Sietze de Vries Most organists are aware of the many 'missing pages' in Bach's plans for his Orgelbüchlein, but few modern organists have the skills and confidence to take on the challenge of filling in the blanks.  However Sietze de Vries, organist and presenter in a new DVD/CD boxset from Fugue State Films, does exactly that - and he does it through improvisation rather than creating laborious written-out scores. Sietze de Vries is an international concert organist and church musician, renowned for his improvisation skills. Looking at Bach's plan for the contents of the Orgelbüchlein (a…