The isolation pad is step one. Step two is buying your neighbor a bottle of JD and some ear plugs.
Your basement/home theater goes under his first floor? What kind of ceiling do you have? Drywall? Tee-bar? Can you access the 3' space? Some pic's would really help here.
To reduce the transmission of low frequencies, additional mass works best.
Extra layers of 5/8" drywall is how it's usually done.
I live in a condo and run a couple of 10" pee-pee subs but they are plugged into a switched outlet. After 8 or 9 PM I switch them off and before 8 or 9 PM I'm careful with the volume. While watching movies I keep it turned up enough so that dialog is ... I can't think of the word ... understandable? Even then if there's a chase scene with crashes I'm likely to reduce the volume on the fly for that portion.