Having trouble with sound deadening

I'd suggest reading Premium Home Theater by Earl Geddes. It's well written and easy to understand, and it nicely lays out the theory of room acoustics and specific solutions. You've already hit on the major problem of the structure transferring vibrations to other rooms. I think the most effective solution would be to incorporate resilient channels into your walls and ceiling that physically decouple the sheet rock from the studs, beams, etc. (floors could be a tougher problem if not on a cement slab). Not a small job but not horrible either. If you're handy you could probably tackle it yourself with a helper. Or, alternatively, the book is detailed enough that any competent contractor should be able to do it (i.e. you don't necessarily need someone who specializes in audio rooms and is really expensive). Either way, I suggest reading the book because it will help you understand the underlying issues and probably with the rest of your room's acoustics as well. I found it extremely interesting and educational, and it was by far the best of the several books I bought on the subject (at least for a layman like me). Best of luck.

For $50 you should try the Auralex subwoofer platform.

I recommend a couple of bass shakers. Too much bass from powerful sub could hurt listeners’ ears. Young people's ears are easy to damage! The bass shaker is more effective and fun to feel the action in the movie. They are quiet, small, cheap and easy to install. Bass shakers should be attached under the sofa or listening chair. Then you just need no or one small powered sub with low volume at night.

Look up Quiet Rock_ to add t your existing drywall. Do not remove existing drywall- waste of money. add to what you have. Resilient bars, with quiet rock---incredible improvement!!! 2 or three treatments will stop train rumbling by!!!

Bass moves in mysterious ways in a room. There are almost always peaks and nulls that occur in a particular space. One thing to try is to move your sub or subs to a different part of the room. That can change the way bass energy is transferred to the rest of the house. Also try them near field closer to the couch. That way you can turn down the sub volume and still get the bass response you are looking for.