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SimpleSE R17/27

It shouldn't. Those resistors are just carrying DC. The idle current through the tube passes through that resistor, but any AC (audio) signal should be going through the bypass capacitor, C12 or C22.

Metal film would be a poor choice for this position. Metal film types tend to catch fire and burn if you happen to exceed their power handling limit. This resistor needs to be able to dissipate a fair amount of power. Metal oxide types are often rated flameproof, but again you need to ensure you've got sufficient power handling capability. KT88 type tubes can draw a lot of current, and you really need a minimum 5 watt rating for the cathode resistor. It might be difficult to find that in a metal oxide type.