Getting a "rubbery" sound with instrument decay or any other aspect of the music is unusual with UltraSonic SS Damping Instruments. Might be interaction with a complementary aspect elsewhere in the system, like Sorbothane or other resonant damping material, or even stock rubber feet. Experimenting with placement of the damping instruments on the tubes might give a better result, closer to the base or closer to the top of the tubes, for example. Or, could very likely be the SS dampers are still breaking in (takes a while for the silicone pads to fully cure, especially with cool-running preamp tubes).
Introducing other materials into the vibrational environment will virtually always give a "tweak" to the sound. Though UltraSonic SS Damping Instruments almost always give the best results as is, there's no harm using silk or aluminum foil around the isolation pads, or Teflon plumber's tape. As each system is unique, results of such tweaks will often be unique to the particular system. Experimentation is often the mother of invention, so there's no need for anyone's approval.
Herbie's Audio Lab