With the S300 the single biggest thing you can do is add a set of bypass caps to the two main filter caps. This will be bigger than a power cord. If I remember, the rail voltages on the S300 are 80VDC, so you would want to use 100VDC caps as a min. Try a 3.3uf, a .47uf and a .01uf in parallel on each caps. Parts cost will set you back about 6 bucks.
The S300 and S500 benefit greatly from parts upgrades, especially the resistors if Vishay bulk foil are installed, but that is a story for another time.
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