Hi Philippe,
Like you, I once lived in a "semi-detached" row house in Toronto and I had two large subwoofers. There was brick between each housing unit, but it wasn't enough, sadly, to prevent the deep subwoofer bass from occasionally annoying the ladies next door. Deep bass travels horizontally through the structual units of a dwelling, so unless offset studs and other building ploys when the building was constructed are used to prevent transmission to your neighbor, there's no way to kill it.
Your only option that wouldn't be ridiculously expensive (like building a room within a room) is to lower the subwoofer volume. I suppose you could try butt-kickers but I find those kind of gimmicky. Users like them, however, and they do give the impression of powerful deep bass reverberations.
Regards,
Alan
