Is your amp dusty in the least? With tube gear I've found that a very light coating of dust promotes the build-up of static electricity and when turning off the amp it seems to discharge the static electricity and give the symptoms you describe. Get an anti-static dust-cloth of some and give the amp and tube a wipe. I always go over the tubes with a damp (Windex) papertowel when finished to insure nothing is left on them that could create a hot-spot. YMMV.