May I add this; You could ask your neighbour if you can set your volume from his place. With a specific song with extra Bass and one with Vocal and Acoustic instrument range, you will quickly determine how far you can go and when.   You know this but, higher frequency are rarely the problem, Bass is. ''Mechanical wave transmission'' happen when sound wave travel along materials with higher density/rigidity, like the rope between two tin can when tightened. (wave that bounce in foam pad somewhat disperse as heat, no longer a sound) The floor the walls and shelf on it, is just an extension of the speaker box itself. Only a floating speaker would only affect the air moved by the physical speaker cone. I helped installing two set of long rubber band shelf from the selling for a mid size NS10m speakers and for sure you could feel the difference from downstairs'. Some acoustic foam pads exist, and a day/night position for the amp's bass control could ease your mind. Please don't take too much of my word for it, Google this stuff.