Peter,

A rigid room boundary is generally preferable

I couldn't disagree more. The best wall material for a listening room is
probably something like 1/8th inch plywood. This way the bass would pass
through to the outside, while the mids and highs would still be reflected so
the sound isn't completely dead. Here's a graph showing the low frequency
response I measured recently in a typical 16x10x7.5 foot room:

www.ethanwiner.com/misc-content/response.gif

The severely skewed low frequency response is caused by reflections off the
walls, floor, and ceiling. If the sheetrock walls were progressively
replaced with thinner and thinner materials, the size of the peaks and nulls
would be correspondingly reduced. If the walls are eventually made as thin
and light as paper, the LF response would be completely flat.

The other great issue in the bass range after flexing floor/ceiling is
flexing window panes.

Why do you think flexing is a problem? If anything that equates to bass
trapping, which flattens the low frequency response.

--Ethan