My intention was to check the audibility of second-order distortion on music, which already contains harmonics. The maximum setting (-12dB) corresponds to 25% THD which is insanely high in terms of "modern" equipment (DAC, preamp, amp, ...) performance.
Currently, I do not have any decent...
To test the audibility of pure second-order distortion I have written a Foobar plugin which simulates a quadratic transfer function. On the configuration dialog of the plugin, you can set the desired level of the second harmonic in 24 steps between -150dB and -12dB and also turn it off...