I'm puzzled, because the gain setting of the phono stage would have zero to do with static, and I cannot think of any other electrical phenomenon that would cause the noises you heard or hear. Unless engaging the 67db gain setting switches into the circuit an additional gain stage within your phono, and there is a component of this high gain stage that is faulty. That situation would have nothing to do with static electric charge. But such a glitch should not get past your mute switch. So, there is something inconsistent.
I have a feeling this is not the end of the story, but I hope I am wrong.