Thats actually one thing I think the original grounding scheme got wrong, instead of the transformer center tap connecting to the power supply cap ground, it lands a fair distance away from it. Somehow it managed to be quiet anyway, but usually you want the first cap ground at least to be connected to the same point as the transformer so no hum current has to flow across the chassis. Using a cap with a ground terminal and running a wire from there to the trafo CT fixes that issue.
