Disklavier

Hi folks,
  I have a 5.5 year old Mark IV in which the I get no sound from the bottom two octaves when in the silent mode.  The first key that plays is the third C# from the bottom.  I get the same result from the headphones and speakers.
  Any ideas on what's gone awry and what I'm looking at in repair costs?  Also,is it possible to get a nicer sounding grand piano in the silent mode?  I've never been happy with the electronic grand piano sound.
  
Thanks in advance,
Steve

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