I have tried speakers on stands, sitting on subwoofers, rubber sheets, foam wedges, and various combinations.
They all sounded the same, and measured within margin of error. Well built speaker cabinets aren't transmitting enough energy to need decoupling, and the same goes for subwoofers.
By far the most important thing is your speakers sit at ear level. Most bookshelf speakers have only 10-15 degrees of vertical listening window.