Welcome!
Isolation is a rather complex subject. I worked as a structural engineer for 8 years and along the way had an opportunity to associate with some super smart guys who designed concrete pads for power plant fans/motors. The design involved dynamics (vibrational analysis). Learned some basics from them. For one, the type of floor (soft - wooden, versus stiff - concrete) makes all the difference in proper support. For another the weight of the speaker/stand and it's operational frequency range also makes a big difference. Vendors that don't try to take those factors into account should be avoided. Isolation products are definitely not "one size fits all".
By "shelf speakers" I assume you mean monitors on stands. Ideally stands should be heavy and have a broad base to add stability. The last stands I bought came from Timber Nation. They weighed 25 pounds each and held 50 pounds of sand. With the speakers each side added up to 100 pounds. In the case of stands you'd have two sets of isolation needs: one between the monitors and the stands; and secondly between the stands and the floor. Same idea for turntables on racks (two sets of isolation needs exist). Speakers and turntables on soft floors need very soft isolation so the resonant frequencies of the floor doesn't coincide with the operational frequencies of the pieces in mind.
