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Ah that changes the picture! I thought you were trying to repair it.
So you want to redesign it to eliminate the hum. One thing you can do is cut off the low end of frequency response. Is the hum there with volume set at zero? You can add a bit of feedback (degeneration, actually) by removing the cathode bypass capacitor on the output tube.
You can also attenuate the audio with a small network at the voice coil; this will reduce maximum output level but you want it for quiet listening so that shouldn't matter. About 10 dB ought to do it, maybe a bit more. So the hum will drop that much, and the maximum output power will also, down to perhaps about 100 mW, plenty for your needs.
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