I never tested the amps and I wasn't there, so asking me to explain the sound quality differences is a little strange.
But you read the article, right? So what was the authors' explanation for the differences they heard?
Clearly something sounded different, and if there was a frequency imbalance surely the experimenters would have documented that.
Did they document that there wasn't?
Did you see anything in the test to suggest a frequency imbalance large enough to result in audible differences?
I haven't read the whole article. You did. Why don't you answer your own question? What was the reason for the difference?
Perhaps you should claim the results were wrong because they contradict your preconceived notions about amplifiers.
My "preconceived notions" are in fact supported by peer-reviewed science texts. Please explain how the results of that test contradict them.