I don't think there's a darn thing dampers can do to extend the life of tubes. Yeah I suspect vibration can shorten the life of tubes (just as vibration can shorten the life of light bulbs), but we're not talking about guitar combo amps here so the type of vibration we're getting is not severe.
In my experience what tube dampers can do is reduce sonic smear and overhang, for want of more precise terms, in some contexts, by reducing acoustic breakthrough. Some people might hear that as a reduction of liveliness, but it's an artificial, detail obscuring "liveliness." That kind of distortion -- the zinging, ringing "liveliness" of acoustic breakthrough via microphonic tubes -- can be good in guitar amps where distortion and coloration of the signal is the goal (though I use tube dampers in my guitar amps), but I for one try to keep it out of my hifi chain. And since all tubes are microphonic to some extent, tube dampers can help.
I'm among those who quite like the Herbie's dampers and haven't found any thing else to be effective -- no point in using, say plain silicone o-rings, to me they don't do a thing. I having tried the brass things that go on top of the tubes but I have friends with ears I trust who like them. But of course none of these solutions can make a badly microphonic tube that's howling into one that's useful.
