[JohnM]

You can look at the scope tab to see whether the input signal is externally clipping, if you are leaving your meter on the 80dB range for all those tests then the meter will be the limiting factor. If the meter needle reaches the upper end stop you need to increase the meter range. You can either use a higher meter range for all the tests and live with reduced signal/noise at the lower levels, or switch the meter setting as the level increases and offset the measured curves accordingly (though the steps on the meter will not be exactly those indicated depending on component tolerances).