If one tube is redplating and you have tried the obvious things like proper Mesa range tubes and all good connections at the sockets, as it seems that you have then I would suspect a problem with the bias supply on that tube. If you know what you are doing re safety precautions (there are lethal voltages on some of the pin/sockets!) you could try measuring the bias voltage on the grid of all the tube sockets with the tubes out - if its Simulclass the two outers should be the same and the two inners the same but outer is different to inner. The plate resistor is another possible problem, these can be damaged during tube failures as Mesa don't fit an HT fuse so these sometimes act as the fuse when a tube fails, checking the plate voltages would probably show up any problem. Do you know that the amp was working all ok when it was last used 9 years ago or might this be the reason it was out of use? Another possible is a capacitor problem due to the years of disuse. I believe that it is recommended to bring the voltage up slowly to charge the caps on an amp that has not been used for many years - but it seems less likely to me as I cant think why only one tube would be affected. Sounds like the amp needs a proper check-over to me, its unlikely to be anything very serious/expensive I think. There are schematics on the internet that show the various voltages. I expect someone on here could expand on this - I don't claim to be a real expert.
