Hi Chess,
I forgot to say that to interpret those THD charts, you subtract the number of dB between the SPL level at the top (usually around 90 dB SPL) and the distortion curve at the bottom. So if there is 40 dB difference, or, to put it differently, if the THD curve at the bottom is 40 dB down from the upper curves, the distortion is 1% or less. If the distortion is just 20 dB below the test level, then the distortion is 10%.
As to midrange audibility, if you keep THD at 1% or less, it's generally inaudible with musical programs but audible with a fixed tone.
With music, THD distortion usually has to get well into the whole percentages to become audible or annoying. And this is even-order harmonic distortion, which is "musical" in a sense, nothing like as nasty as odd-order distortion that you can get from amplifiers.
Regards,
