Sounds like you already know what you need to do; replace those tubes - cheaper than a woofer!
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I would (obviously) test all tubes and replace as needed (keep track of the performance specs for each old and new tube that remains in the amp). After doing this I would wait a reasonable amount of time (say three months) and then recheck the tubes again making special note of the tube from "that" socket. If "that" tube's specs degrade considerably more than the others, I would then have the preamp checked out by the manufacturer or another proffessional.
Gthirteen- I doubt that it is simply a "tube" problem. It sounds more like the bias on the suspect tube is off. You can try swapping the two tubes, to see if the problem stays with that particular socket. Also, simply reseating the tubes (cleaning the contacts upon removal and reinsertion) may help with dirty contacts. Don't give up on tubes. SS is more convenient, but rarely as musically rewarding. Just my $.02. Goodluck.
Rethinking this one a bit: it's quite possible that the tube is going south due to some other component-failure or value-change, such as resistors / caps. If contact cleaning, tube replacements & biasing don't cure this, then have the suspect unit bench tested. It sounds to me like you're not even for sure if the trouble is in the amp or preamp? If you don't know for sure, you might try leaving the amp running with no input connected, for a long term test. Good luck with your troubleshooting G13.