I think you have already followed the no sound trouble shooting steps - nice! Just double check you got all points,
As you have already changed the tubes and know the fuses are good, there must be something else wrong putting a break in the signal path. It could be simple as a switch. So any of the other impedance or reactive load settings change the no-sound condition?
It might be time to think of getting a service for the unit. Check the FAQ category for options there.