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I've sorted it. It seems it wasn't a parasitic oscillation after all, it was simply hum from the relay coil. Switching the relay round so that the coil is energised when the extra gain stage is switched out of the circuit has made all the difference. I didn't have time to check it at louder volumes this morning but I will later just to be sure it's all gone. After all, there may well be more than one source of noise.
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