The main problem I see is the Intermodulation Distortion (IM). It is generally 3.2 times that of Harmonic Distortion. So if an amp peaks at 3% distortion, the IM is close to 9.5%. If it is 5%, then the IM is more than 15%!

An example of IM distortion: a 1k and 800 signal "mixes" to produce signals of 200hz and 1.8khz. No wonder the music might seem filled out.

One can tell the quality of an amp when a "review mentions liking simple music and not complex, like orchestra. This tells me the IM distortion is very high as it really messes up the music.

Another area is the drive needed for the output signal. Alot of distortion comes from the driver and even before that. This mixes to produce higher order harmonics, although lower in amplitude, that are easier to hear.

Passive parts distortion also plays a major role in the sound. Using passive parts like carbon resistors is guaranteed to artificially "distort/color" the sound.

So the problem with an amp is much more than HD, but IM, topology, parts distortions etc.

Steve