The problem with any discussion regarding measurement and audibility of harmonic distortion is that our sensitivity to harmonics varies with what order distortion we are hearing. Even order or low order harmonic distortion can be difficult to distinguish at high volumes and may even sound pleasant. For example, many people would be surprised if they knew how much second order harmonic distortion they listen to unknowingly with their favourite subs. Tube amps in particular have high levels of low order HD which is why many people find them appealing.

The offensive stuff is the high order or odd order harmonics. Very small amounts of odd order distortion can be detected at low volumes. While the 1% THD stat is frequently cited as a threshold amount for amplifiers, especally with car applications, what that stat ignores is the composition of THD and the fact that each of us has different sensitivity to HD. I often hear my tube amp friends talk of the "harshness" of solid state amps which I interepret to mean their sensitivity to odd order harmonic distortion. Listening test studies have shown odd order harmonics in the 2khz to 8khz range can be detected as low as .01% by some individuals.

I would strongly recommend not buying any solid state amp which is spec'd at 1% THD without knowing how much of that number is odd order harmonics (though not many manufacturers would dare cite that that number). Also try to determine what the THD curve looks like when graphed against frequency response. THD will also vary with frequency and manufacturers will often cite the most favourable looking stat.

John