Many zoom lenses are soft at the extremes of their ranges.
1/80 is a fairly very slow shutter speed to be using at 400 mm.
A general rule of thumb is the shutter speed should be at least 1/the focal length of the lens for hand holding.
You do not state the make of the lens, if it has an image stabilizing feature and if the stabilizing feature was or was not used, the lens aperture used, nor if the lens was hand held or tripod/monopod mounted.
Since it was a 'dark concert hall' it's also likely the lens was used at it's widest aperture, another 'at the extremes of their ranges' setting.