Audio Science Review of Expert 200.

[thumb5]

... the amp does not have to be capable of delivering rated output for a sine wave signal at any frequency in order to meet its rated output spec with a broadband signal or with music.

While this is true for all practical purposes (listening to music) as you pointed out, apparently the reduced power output above 10 kHz violates the US FTC standard for home audio amplifier power specifications.  Amir points to clause 432.3(e) which states:

(e) Rated power shall be obtainable at all frequencies within the rated power band without exceeding the rated maximum percentage of total harmonic distortion after input signals at said frequencies have been continuously applied at full rated power for not less than five (5) minutes at the amplifier's auxiliary input, or if not provided, at the phono input.

That seems fairly clear, so at least in the US it could be argued that the measured behaviour of the 200 is a regulatory concern even if it is of no practical consequence.

Personally this doesn't bother me at all from a listening-to-music point of view, for the reasons you summarised, although I would like to understand the technical rationale for the behaviour that several reviewers have now noted.  My hunch/guess at the moment is that limited power delivery at high frequencies is an inherent "feature" of the ADH architecture and the amp is protecting itself rather than the loudspeakers.