What problem are you trying to solve?
Transmissivity of stylus tracing noise into the platter ?
Which implies transmissivity of floor or table noise that got into the platter then being transmitted into the record ?
How about a peek at "Noise & Vibration Control", Leo Beranek, graph on pg 412 for starters....
Or- better record to platter isolation?
Stylus tracing 'noise' from the record groove would seem to be very low.
So you have two possible causes for the change ?
VTA and damping/isolation introduced between the platter & subplatter...
Reset the VTA to original and then you will have a better idea. If the cart had been flat w/r/t the record then a mirror may help.
There are many types, shapes, and durometer of 'rubber' you could try but I would answer the VTA question first.