Can you tell me exactly what settings you made on the Manual Sound Sets page?
What output do violas, cellos and basses go to when you hit play?
For the orchestral strings you need the Orchestral Strings Programs (Violins Orchestral, Violas Orchestral, ...). Also don't forget that if you have one section more than once in your score, you will also have to reserve two channels for it in the manual sound sets page.
Andi
Vienna Symphonic Library