Very often, amplifiers have increased THD+n in the 5/10/15kHz spectrum once power increases past 1-5 watts.
Two things:
If you've got one watt of 10kHz on top of twenty watts of 1kHz, do you get 10kHz @ 1 watt performance for the 10kHz tone, or
Is the distortion of the 10kHz modulated at 1kHz, increasing to a peak of 10kHz @ 20 watts at the crest of the 1kHz wave?
That was the first thing - it had two parts...
Second thing:
Typically
What does the 10kHz FFT look like@ 1w, 2w, 5w, 10w, 20w, 50w?
Are the harmonics the same, or do they change?
Does noise increase?
If these things don't follow a pattern, let me know.
Also, does class AB typically behave one way, D another?