ground loop thru cable co-ax? Try ground isolation transformer on the co-ax. A good trial is to put two of those 300 ohm to 75 x-formers back to back. co-ax to 75 ohm to 300 ohm to 300 ohm to 75 ohm to co-ax.
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Is this the only digital input being used on his pre/pro.
If it is try a digital connect to his CDP, it may be his Pre/Pro. One other thought is phone line, did you hook up the phone line as DTV so loves us to do. Disconnect it and see what happens, but my first thought is the Pre/pro is generating a hum withing the digital to analog circuitry.
Not an expert here just have had DTC for about 12 yrs and this is a thought of something to try.