I lived with mine for over 20 years with my ET arm and tried several things.
It has a very resonant base that is difficult to overcome. It is a tweekers tt though. I’ve used the SIMS, practice golf balls, etc... In my experience, anything rubbery/squishy became a trap that destroys ultimate dynamics, transparency and bass response. ( I believe this to be a general rule of thumb. The 1st thing I did when I bought my Classic 3 Sig SE was to replace the rubber (inside) feet with Stillpoints SS). I was never happy until I ended up using a stack of Symposium Rollerblock Jrs. w/ ceramic ball bearings, that I had to modify the base for their use.