I was puzzled by your question…. checking with the original circuit diagram, the output gain control does not shut off completely…. there is always an element of output, but it is all ‘processed’ signal, coming from the input amplifier via the valve stage, the compressor and the enhancer. That’s the way it is.
At that time I always designed so that the input and output controls worked more as ‘presets’ and did not turn off completely.
So the answer is, if there really is any signal going directly from the input amplifier to the output stage, then there’s a very unusual fault somewhere and I have never come across that before. 