Nyal,
You can measure different decay times for different frequencies, and using 3" absorbers still leaves the decay times unbalanced. The question is, is an unbalanced RT60 really disturbing given the fact that the sound field in the case of directional loudspeakers in a small room is highly directional?
According to research by Bech with speech and noise it is the floor reflection that has the greatest impact. According to Guski the ceiling reflection may cause confusion when being the only reflection remaining. Was the ceiling reflection the only or merely one of several reflections to be treated? You say that the back wall reflection does not do anything beneficial, but is it disturbing? With music Wrightson found that a rear wall reflection did not change the image width significantly.
Since off-axis response is responsible for how the reflection will look like spectrally it would be interesting to see how the off-axis responses of the systems you’ve treated looked like. The cases you are mentioning may have presented reasons that made reflection treatment have beneficial effects. Were the systems 2-channel or multi-channel? I’ve prepared a 23 page literature review (88 references) on this issue of early reflections (2-channel only), if interested, send me a note.
Klaus