We have very good walls here. Despite being surrounded by neighbors, I never hear anything. No voices, no washing machines, no tvs, nothing. The only sounds are footfalls from above at times, but those are transmitted via the frame, they aren't direct sound.
That said, my main interest in accoustics is to maximize the fidelity of the system. However, if I can also keep some of the subbass inside, that will be a plus. I will be using isolators and also experiment with placement and phase with the two subs. Daytime listening isn't much of an issue, however. I can be reasonably loud no problem.
Here's my simple dream: to be able to run the full system at night, with subs, at about an 80dB level with nothing but the slightest muffled throb coming through, if anything. I think it can be done.
Anyway, it remains to be seen what can be accomplished. I've very qualified people in other places who can help me with that

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No spirits dampened here. I only get more and more excited as I think about it all.