What's this test illustrating again, I think I remembered from an earlier post.....it's showing that the 2nd Harmonics introduced in the amplifier are destructive when it comes to real music rather than synergistic....something along those lines?
Standardized 'normal' Intermodulation Distortion tests only use two frequencies to see IMD. There are a number of standard frequency pairs for this to test capabilities at different frequency ranges. The purpose of a multitone test is that it's meant to be reflect 'real music' (you know, that excuse which subjectivists always bring up because they don't understand that their music is made up of a sum of sine waves anyway) by using more than two tones. It's also more convenient than having to run numerous different frequency pairs.