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Sure you can use zener diodes but they would have to be in series with the high voltage diodes. You'd need a heat sink.
If you save 15 W per 5U4 that could be a very nice improvement, allowing the transformer to run cooler, reducing the heat rise in the unit, and of course drawing less from the power line. That savings is independent of the rectification loss, which you have in either case if you size the resistors properly.
No, the resistor won't waste more than the 5U4. If you do it right, it will be exactly the same as the tube. Just the filament power is no longer needed.
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