Going on Getzners diagrams the common 12 mm thick sylomer gets down to about 17 Hz natural resonance if properly loaded. It should then isolate from 17 x (root 2) = about 24 Hz and upwards and start to amplify vibrations below 24 Hz, with roughly 18 dB amplification at 17 Hz. If the speaker load is too light on the sylomer the natural resonance goes up, giving a poorer isolation frequency wise. Too heavy a load and the rubber "bottoms out" giving a stiff(er) connection instead of a floating one.

To reach a very low natural resonance with the sylomer, you have to go for a thicker version than 12 mm.