Here is another question. I used a product called ensolite in my car, It is a closed cell foam about 1/8 thick. if i remember correctly it helps in deadening outside road noise from creeping in. i used it on the floor and the doors of my car. i can't tell you if it made a difference because i never heard my sytem without it. My car sounded great. Would a couple of layers of ensolite under the sub help any?

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).