My friend's $$$$$ Dr. Z Monza (that is an EL84 amp, and unreal loud) suffers from bad tube rattle. As you've said, some amps do and some amps don't. I have a Crate V-18 that I can crank and crank and doesn't have the slightest issue with tube rattle, never has. The super expensive Dr. Z sounds like nuts and bolts rattling around inside the poor thing (shame, its one hell of an amp!). The dampening rings may help though. Glad the tone from the tubes is pleasing though - hopefully you can get the rattle situation worked out with the dampeners! At least now you have a few tubes to play around with your tone when the dampeners come in!