I have no real insight into Devialet design so it is difficult to explain the difference. From the public information that I can see, they put class A emphasis on the output stage, whereas IDA-8 focus on the input stage.
There are many ways to optimize the circuit at this level, and THD+N below 0.01% is not noticeable. In fact, more even harmonics might have worst THD+N but sounded better. Ultimately at this level of performance, it is difficult to judge whether one design sounded better than another. We are really into the PERSONAL PREFERENCE territory. A few years ago, we can pick on some design imperfections in certain design and implementation, but it is harder to do now.
I think Devialet, Hypex, IcePower and NuPrime can all come up with very good designs. I can comment that within NuPrime, we are now constrain not by knowhow, but by cost and sound characteristic. I am not saying that there is no room for improvement - for example, we are looking into having multiple personalities integrated into one amp, or controlling the even harmonics.
I would also say that in the past, the challenge is how to make it better. Now the challenge is how to make it cheaper (with world class performance). So I think IDA-8 by itself is a benchmark. It is not as good as IDA-16, but unbeatable at this price.