It makes
a big difference what kind of music is used for the test. In another thread I showed an effect of cross-over like distortion to narrow band noise
(1000 - 1300 Hz). Though the distortion used seems quite low on the usual 1kHz high level sine, it is quite devastating for the low level signals.
and the distortion used is shown in the plot attached. If this distortion is applied to a
rock music (I tried well recorded Godwhacker by Steely Dan in 96/24),
it is almost impossible to tell that the distortion was added.
If I added the same distortion to
Beethoven's No. 9 symphony Movement No. 1, the
distortion is immediately audible in the low level passage just after the beginning.
So the
test files should be chosen very carefully and they should contain
high dynamic range and both loud and silent passages, the music that is always close to full scale level is not good for testing.