start from swapping tubes.
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John Atkinson's measurements indicate relatively poor channel separation for the 912, which is suggestive of the possibility that isolation between different inputs may also not be very good.
Yogiboy's suggestion of shorting plugs on unused inputs could conceivably help. Rearranging which inputs are used for which sources could also conceivably help. But I wouldn't be surprised if the only solution is to turn off or mute inputs from sources you are not listening to.