Thanks! It certainly appears from the XBA-3 graph that the higher the output impedance, the more V-shaped the sound gets. Since some people do complain of metallic or sibilant highs with the XBA-4, and the observation that high frequency becomes increasingly uneven as output impedance increases in the XBA-3, I think we can make a guess that the XBA-4 sounds less shrill with players or headphone amps with minimal output impedance.
I think I know what people mean about "metallic treble" but I'm not sure that it is necessarily beneficial to aim for a lower output impedance and therefore a warmer sound signature, which could be considered throwing the baby out with the bathwater. I might have to do some more listening with classical and acoustic genres to see if my attitude changes, but for the heavy metal music in my collection there is nothing really wrong with metallic treble as the instruments are meant to sound metallic. If violins sound steely though, there might be problems.