. . .when people install sound systems in basements where all sides are concrete, it is best to install acoustic wedges or other absorbent treatments and panel dampers on the walls to damp room modes. Multiple subs are always a good idea too.
Is that meant to contain echoes, to keep the sound from getting distorted, or to keep the sound from leaking upstairs? Do you think it's worse when the flooring is concrete too? I have a system in the basement and it's hard to hear it. But then again, it isn't an expensive setup. Just old gear.