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A 6CB6 or 6BZ6 is a reasonable substitute for for the 6DC6 for testing purposes. The tranconductance is a little higher for these tubes (8000umho as opposed to 5500umho for the 6DC6).
Of course you would not want to mix up the existing tubes or necessarily permanently exchange the IF tubes as realignment would be advised. But I would try a little step by step substitution to see if the hum clears up.
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