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Have you replaced the selenium rectifiers with diodes? If so, that sometimes results in some 60 Hz (actually very high harmonics of 60 Hz) noise that is hard to get rid of.
In one set where I saw this exact problem, putting a 0.1 uF capacitor across each power supply diode did the trick. This functions as a snubber to reduce the rapid and harsh switching in a modern silicon diode compared to the original selenium rectifier.
You could also try bypassing each electrolytic in turn with a 0.1 uF cap to help quiet down 60 Hz spike noise.
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