David_Ten and Ron1264:
Used pillows rather than a stool in the same way and noticed the improvement. This led me to design and construct some wall panels that have cleaned up the issue entirely.
Have found that Herbie's gliders not only tighten up the bass but, because of the added vibration control, contribute to a more accurate soundstage. And, as well, if you damp the wall behind the soundstage area (= wall space between the two DIs) then the depth of the stage will open up as it does not have to compete with reflected waves.