Sound travels in the air and is also conducted by floors and walls. A pad of some sort can cut down on the conducted sound, but will not hinder the radiated sound. You can get a clue about the the radiated sound transferring through the wall by listening for any neighbor’s audio leaking through to your side. Also note that sound can sneak out through windows and enter the windows next door.
The sound shouldn’t be too bad for your neighbors, I’d suggest getting the Sub and trying it.
Most isolation pads are badly overpriced for what they do. Drop by a carpet installer and ask for a couple scraps of their better quality foam under-carpet pad and you’ll be as well off. Cut them to match the area where the Sub contacts the floor.