OK to replace one power tube for amp?

Brf, Speak for yourself :)

Good advice on buying new tubes and compare. This is my phylosophy except I leave the new tubes in and keep the old tubes as spares.

With only two output tubes I would replace both. In fact if the tubes have a lot of hours I would replace tubes in both amps if you want the best sound. Tubes wear slowly and we adjust to the sound the same way we adjust to our car when the shocks wear out. I think you will be pleasantly surprised at how much better your amps sound with new tubes. If your amps are push pull the weak tube is placing more wear on the strong tube and wearing it out quicker and since the tubes are not holding equal bias the sound quality is compromised and causing the distortion to rise.

Most manufacturers estimate tube life in their equipment.