I use Herbie's on the input 9-pins of my VTL mono's and like what they do, regardless of whether the tubes seem overtly microphonic or not (as determined by the tap test). The dampers help with better articulation of vocals, for instance, with less sibilant edge and more palpable image focus/dimensionality; give a clearer, 'calmer', more naturally stable soundstage that's less perturbed by what's going on musically; lend better purity, indivuated tonality, and detailed realism to cymbals; and generally reduce a slightly forward excitability in the presence range, making listening easier at higher volumes. As these symptoms are all things which can be attributable to resonance in tubes, I'd say the dampers do their job -- you hear the tubes less, the music more. I don't think this effect is necessarily unique to Herbie's product vs. the soft rings made by others, but his are probably easier to move and remove, don't bind to the tube, look nice (if your tubes are in view), and still fall under the catagory of "cheap tweaks". Not a dramatic improvement for me, but easily more than worth their small cost.
When I ran a tube preamp, I didn't have the Herbie's, but tried Pearl tube coolers with the woven graphite undersleeves. This product probably works for reducing heat build-up (I also used them on some power tubes, but can only guess at, not measure, their effectiveness), however I wouldn't look to the Pearls as an anti-resonance treatment for small-signal tubes. If anything, they are prone to making 9-pins sound more resonant, not less, and these tubes generally don't produce enough heat to make any reduction worth the cost or sonic tradeoff. No audible problems when used on power tubes though.
