I can't comment on the neighbor part because I haven't had any in shaking distance for a lot of years but I can comment on the difference between having no sub, a cheap sub, and a 'great' sub even at low levels.
When I bought my M60's and VP150 I waited a few months before ordering the sub to build up the cash. In that time, I watched movies without a sub until I unpacked my old wimpy sony one from my old HTiB system.
My wife doesn't like movies loud like I do, if they are action oriented. Adding the little sub added a bit of fullness but not so much at low volume levels. You could tell the difference but you were not hearing the extreme lows.
But!! When I added the EP500, even at low volumes it did so much more. The deepness came out very full. It didn't vibrate your world at low levels but the difference was significant.
The differences at high volumes were vast. It created lows that would make the hairs on the back of your neck stand up. However, it did so without drawing attention to the box itself. You could crank the old Sony for sure but it mostly rattled and drew your attention away from the movie as your eyes tended to look at the sub because it became very directional.
NO advice on what to do, just sharing info to help you decide.
