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Masking of intermods vs masking of harmonics

First of all, thank you Tom for the explanation of masking. It's actually the clearest explanation of the topic I've read yet. I think I understand it now. However, I'm still bothered by what seems to be a contradiction when thinking about single tone tests vs multi tone tests. Let me try to explain.

Suppose you have an amp that has only second, third and fifth order distortion. First consider a single tone. Certainly the second harmonic is closest to the fundamental and is therefore most easily masked. The third is less easily masked, and the fifth is even less easily masked. Okay so far.

Now consider two tones at, say 10 kHz and 11 kHz (call them f1 and f2 respectively). The second order distortion of the amp produces the difference tone at 1 kHz or f2 - f1. The third order distortion produces a tone at 2 * f1 - f2 = 9 kHz. The fifth order distortion produces a tone at 3 * f1 - 2 * f2 = 8 kHz. In the harmonic distortion case, the lower order terms are closest to the fundamental. Yet, in the intermod case, the second order term is way far away from the two main tones. The third order term is closest, and the fifth order term is further away than the third order term, but both are closer than the second order term. So the masking concept, when applied to intermods, seems to favor the third and fifth order terms over the second order term.

To me, this seems to be a fundamental contradiction in the attempt to relate distortion orders to the concept of masking. Is there some explanation somewhere that resolves this apparent contradiction?

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