455 Plate Voltage too high for RCA Black Plates?

[Timinator]

I'm measuring with a Weber Bias Rite. The plate voltage is 455 and the tubes are both at 41ma. Is this plate voltage OK or is it too high for RCA's. I've been told that it's too hight the tubes won't last long. The amp is a '67 Super Reverb. Can I lower the voltage, is so how is that done? Thanks, Tim