Is there a noticeable difference in between 0.03 and 0.003% THD for a normal ear?. On what limit this should be an issue to discard an amp as an option?.
And what about S/N ratio. What numbers are good when comparing different alternatives?
THD is generally regarded as inconsequential in an amplifier until reaching over 1% in steady state tones. 3% is acceptable in music and it is more an issue of how the harmonic content of the distortion is put together. Even order harmonics (the type most tube amps produce) are considered more "musical" than the odd order harmonics of solid state amps. MOSFET's, a type of transistor, are used often in audio gear for its "tube like" sound, which is largely a matter of the harmonic distortion content similarities of the MOSFET and a vacuum tube.
Loudspeakers swamp amplifiers in the THD department with most low frequency drivers exhibiting 10-20% THD when driven at reasonably loud volumes with low frequency content beneath 100Hz.
What is considered more offensive than THD is IM distortion. Since its components have nothing to do with the natural sound of instruments, a small amount of IM can be very tiring and lead to listener fatigue. IM is hardly quoted in specs since THD has been pushed by the industry.
Harman Kardon, over thirty years ago, showed TIM (transient intermodulation distortion) is probably the most harmful "distortion" to the accurate reproduction of music.
Forget THD, it matters little until you consider how the manufacturers achieve low THD specs.
It is equally difficult to make sense of S/N numbers. It depends on the circuit that is under inspection and the weighting system used to make the measurement.