5% THD to .000 THD SOUND

Something to keep in mind is that "harmonic distortion" is a way of measuring a distortion from an amplifier, you don’t play music, for instance "at 5% distortion". The amplifier is driven by a single frequency at a time to some rms voltage (or power) output level (typically), under load of some kind, and the level of harmonics relative to the drive level are measured. Music isn’t at any constant rms voltage level, and not at one frequency at a time, so the conditions of the test seldom occur at all, or if they do, they do only for a very brief instant. Usually the harmonic distortion is given at the upper end of the amplifier’s output power capability, as that is where it is nearly always highest, and is given to indicate where the amp might be expected to be running out of juice.

With music, you might hit that peak output power for maybe a millisecond or so, and unless there is a lot of it hitting or exceeding that peak, you won’t be experiencing the effect that mght occur up there. The average power with hifi music is way lower than the peak, so if anything, the relevant distortion at the lower power would be what might be relevant.

The percent distortion spec, as usually given, should be read for the POWER level that produces it (i.e., to indicate what the manufacturer assumes is the maximum power the amp might do). NOT for the distortion level that the manufacturer is calling its limit. An amp with a soft clip can go near that maximum without distressing you because it takes longer to get there, and won’t suddenly go way higher in that region. An amp with lots of feedback (and a low distortion spec to match, used to indicate the power) will usually go from low distortion to high distortion with only a small increase in the power produced. The spec may be 0.0000x% where they call it, but it will probably be 5% or worse with the power level just a smidge higher when the amp clips.

Not making a comment on which is the better amp, just about the "distortion spec". Unless it is given at levels way lower than the amp can do, read it is as a POWER spec.

Interestingly, though, their Overture integrated amp, which is a push-pull EL34 design, has its max power spec based on 1% THD. Just guessing, but perhaps their thinking is that differences in how THD is distributed among the various harmonics when the two kinds of designs

Well, marketing does get into it, too.  A SET amp, particularly one with no or low global feedback, won't have too much better distortion just below the power used to spec the 5%, while distortion of another amp likely will quickly get lower at lower powers.  So, quoting at a lower power won't markedly reduce the distortion figure for the SET, but can reduce it a LOT for feedback class AB amps, and of course that looks better to a customer having only a data sheet to look at.

At the 20dB or so power leve below clipping either amp is used on average at, though, the distortion produced is likely to be low enough to not matter in either case (Geddes did some research showing that for lower order distortions - which pretty much all amps do before clipping - people are just not very able to hear it; other factors are much more audible).

At very low levels, though, a badly biased class AB amp (or one for which the bias doesn't track thermal well) can have quite a bit of distortion relative to the output level due to crossover effects.  And crossover distortion (not speaker crossover -- class AB switchover distortion) is very high order and can be heard.  Class A amplifiers won't do that at low level,  (And, by the way, neither will class D amps.... its a class AB thing).