"Will"?
If the subwoofer is already decoupled from the buildings structure, such an isolation device will have no effect on sound quality, or sound transmission to the neighbors.
If the subwoofer is not already decoupled for the structure, for such a device to be effective, the cabinet/structure interface must transfer enough energy to the building's structure to produce audible resonance in the listening room, and audible structure borne vibration to the neighbors.
And the Sound Transmission Classification of the building's walls and floors must also be considered; even with the sub is decoupled from the floor, the floor and walls will still let some sound through. If the sub is on an isolation pad, but the walls have a STC of 20, the neighbors will still get annoyed.
It is not as though an isolation pad can't, or won't do any good, it's that the effectiveness of one depends on the buildings construction and the subwoofer in question. Without knowing how the sound is being transmitted to the neighbors, you cannot guarantee that buying an isolation pad will be effective.