Help with sony str-de475 cinema surround sound?

Hi I have a sony str-de475 cinema surround sound system and i'm having problems with the sound quality. The center front speaker sounds awfull and the others aren't too great either. I know it's not the speakers because I've just bought some new ones (expensive ones too) I've taken the unit to the shop and when they tried it all was fine? I did notice that when they tried it in the shop the blue digital decoding light came on, I've never managed to get this light on, don't know if it's anything to do with the problem? Please help as I've spent tonnes on this thing and it sounds awfull.