The answer to your question is simple, the Hegel is a better sounding amplifier than the A 21, the A 21 is a fantastic amplifier for the price, the issue is resolution, the Hegel is a more dimensional amplifier with greater clarity and speed as it should be for almost three times the price.
You are mistaking design talent with implementation, sure John Curl is a circuit design genius, but so is Brent Holder and Damien Martin, and Nelson Pass, many other talented equipment designers, all of these amplifiers are good and they all sound different.
I was a Hegel dealer and I am a Parasound dealer, I will tell you there is a new amplifier on the market that I like better than the Hegel and it outperforms the Parasound and that amplifier is the new Aragoon 8008, which costs $4,400.00 which is considerable less than the $5,800.00 Hegel.
The Hegel is still slightly more dynamic, but the Aragon has a stunning midrange quality which is warmer and more musical than the Hegel, which is more analytical, the Aragon has a similar tight punchy bass and is quite open in the top, this is the most amazing amplifier I have heard under $5k and it is a monster in terms of power delivery and build quality.
We used it at the New York Audio Show to glowing accolades our system was one of the best at the show, and it was made up of all reasonably priced components.
So to answer your question, don't just look at the design, the specs or the technology go out and listen.