I have my hi-fi in my converted attic, with Velux windows. It is a 35 square metre room with a 3-metre high ceiling, all panelled in pine. The walls each side are plastered. There is some flutter echo when I clap my hands, but not too much.
The floor is tiled, but I did put a large carpet over the tiles for comfort, between the speakers and my armchair. Not any difference when I remove it anyway.
A lot of treble energy is absorbed