OP,
I think it is very resonable to assemble a high end audio system and only listen at very low levels. I listen at lower and lower levels as I have gotten older and enjoy the nuances better and better.
I do not have an answer. My system has sounded the better it has gotten. Each upgrade cycle has made it sound better at lower levels. I have heard a couple of vary well know reviewers typically listen in the 60db range.