I waited several weeks for them, but I like using the Duende dampers very much on my WE275s. They add a firmness and increase the sense of presence, of coherent realism, without any deleterious spectral effects that I have detected.
Tapping the case of my Stratus causes a zing with most 2A3 tubes, but the dampers dull this effect. Firmly.
They must be engaged carefully, and they fit on the very top tightly - yup, firmly.
Herbie's are good, too, and there are others that I haven't tried. Of course, YMMV.