There are a few things that you might consider trying. Does your amp have enough free air space around it, so heat won't be an issue? I've also read about the pins on tubes getting oxidized over time. Though, I suspect the result would be more of a sound degradation and not necessarily the sound that you've described. However, it still might be worth investing some bucks (about $15.00) in a can of Caigs DeOxit, to clean the tube pins of any potential oxidation. You can find DeOxit at local Guitar Center stores, or even Radio Shack stores.
Give that a try and see how it works out. Just unplug your amp, remove the tubes - carefully, and then lay them down on some paper towels. Spray a few shots of DeOxit on the pins and allow it time to appropriately dissipate and evaporate. I'm sure within 30 minutes, the tubes should be ready to place back into the sockets and give things a whirl. I'm not sure if there are any benefits to spraying the sockets that the tubes plug into. Possibly, another tube owner might be able to shed some light onto any benefits in that area. However, if you did spray DeOxit into the sockets, I'd give it more time to dry and perform its magic. I'd wait a good couple of hours before reinstalling the tubes and trying it out.
Best of luck !
