Heh - that's interesting about the police meter. I had no idea such a thing existed! I wonder if it's similar up here in Vancouver, Canada
Anywho, Rockwool or "soundproof" insulation doesn't do a heck of a lot when it comes to bass. It's mostly just to fill the air void, which deadens higher frequencies a bit, but it's mostly just about reducing free air movement between the two interior surfaces of the drywall (or the drywall and plywood of a subfloor). What makes bass so much more easily heard is that it physically shakes the actual materials that make up your house! It's a bit like how you can put your ear on a train track and hear the train coming from miles away, even when you can't hear the train at all when you're just standing and listening for the sound being carried through the air. That structure-borne sound transmission is the reason why you can hear bass thumping away in a neighboring apartment or house, even when the higher frequencies are completely inaudible.
So stopping bass from "bleeding" is more about decoupling than attempting to "block" it. More material does mean more mass for that sound energy to move, so there is some benefit from simply installing more insulation or drywall or concrete. But the majority of bass "soundproofing" comes from decoupling - which means the aforementioned "room within a room" construction or, at the very least, using visco-elastic constrained layer damping between multiple surface layers on your walls, ceiling and floor (ie. Green Glue or QuietGlue in between at least two sheets of drywall or a pre-made "sandwich" such as QuietRock or QuietWood).
If all your neighbor can hear is the bass - and not the higher frequencies - then that indicates that it is indeed structure-borne sound transmission that is the problem and not just "thin walls" or a lack of insulation that is allowing the sound to travel essentially through the air and reach your neighbor's house. As such, decoupling is going to be your most effective approach. A simple isolation pad is a great, inexpensive place to start. But there's every chance that the air movement within your theater is enough to get the walls, ceiling and floor to shake in sympathy, which in turn becomes that nasty structure-borne sound transmission to your neighbor.
If it is only a handful of specific frequencies that "bleed" through, then those are very likely the resonant frequencies of your particular room. If that's the case, some fairly heavy bass trapping might be enough to curtail those standing waves and reduce the resonant frequencies enough to eliminate the problem.
