Keep in mind that the neighbors are likely not hearing much of anything from the contact with the floor, especially the one upstairs. Isolating your sub will help with sonic accuracy (and I'd recommend you do so anyway), but the fact is that the LF will still cause walls and floors to resonate when it comes into contact with them.

If you can't get into the walls to insulate, there's (unfortunately) no practical on-wall solutions out there that I know of (unless you want your room to look like an anechoic chamber).