Six seconds is used because it has been found in past research that it is all that is needed to make accurate judgements.
I think its possible that people are better identifying good orchestras by sight.I believe the reason is that good ensemble playing is highly correlated with visual cues like synchronized movements. (The bowing of an elementary school string section is usually far more erratic than the military like precision of the Berlin Philharmonic). It's reasonable to believe that good orchestras almost always are more synchronized and for whatever reason laymen find this a more accurate cue than the sounds alone.