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I was fortunate to learn why years later. They are known current hogs, and any oxidation on the sheild of the mic cable (one might be surprised at how much of That exists in studios) translates to undue resistance which causes the "is that mic broken?" question in the first place (no I have not done a detailed failure analysis of the 414).
This seems to apply to all versions.
I know the earliest version's DC-DC converter typically hogged the current leaving little for the amp.