Like Dopogue, I use silicone O-rings, but I purchased them online where I could be sure I got a formulation that would take the heat. They're available very cheaply and in my experience work as well as the Herbie's, the only others I've tried.
What effect they have is, like most things audio, dependent on the situation. I find they make more difference on octal tubes than 9-pin miniatures, and that they make more difference in units that have top mounted tubes rather than the tubes enclosed within the chassis. In some instances, they've made no discernible difference at all, but I still use them because it makes me feel better.
:-)
David
