Devialet Expert isn't good in showing secondary harmonics?

[thumb5]

I took this to refer to reproducing the "timbre" or "colour" of instruments via all their natural harmonics. It is generally assumed that digital equipment often fails to convey it, and can sound 'grey' by comparison. It's one of those things you might not miss until you hear it done properly. After hearing Dan D'Agnostino tube amps at Hi-Fi shows, one can begin to see the point...

OK, I'll bite...

What is special about the "timbre" or "colour" of instruments that somehow makes them more difficult for digital equipment to convey?

I would rather expect that the digital equipment is reproducing the input accurately and that the valve amp is adding pleasant-sounding distortion (typically high levels of even harmonics).  Describing that as a "failing" of the digital equipment would be incorrect in a technical or objective sense, unless confirmed by measurement, although of course it might subjectively be considered not to one's personal taste.

In any case, a blanket statement like the one referred to in the OP seems to me to be trying to portray a subjective opinion in as an objective fact.