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My tube amp also passes sounds when it is in standby mode. This is because the difference between operate and standby is a change from 550v across the power tubes to 300v (and not to zero like you might reasonably think). None of the other voltages are changed. In my case, the 300v is still enough for sound to come through.
According to the designer, the reason this is done is to "condition" the tubes during standby in a manner that does not age them as much as when they were operating. With 300v across them there is a small trickle of current passing through the power tubes which apparently reduces the buildup of oxide on the cathode and lengthens its life. The reduced voltage also reduces the deleterious effects of arcing when the tubes are first powered on since the voltage is applied in two steps.
Maybe your SF is working the same way.