If Warmth Is a Distortion, Why Do So Many Audiophiles Still Defend It?

Any problem with this NIOSH app?

Yes, it is only available on apple products.

 A common experiment is to bypass a stage such as a gain circuit, as in plugging a source into a good passive attenuator and then comparing to plugging a source into a preamp active circuit at matched volumes. The passive attenuator will be more transparent than with a preamp, and also cooler in tonality.

@viber6 I concur with the result (passive controls sounding 'cool' or 'thin') but not the why. 

That is because the passive control allows the interconnect used to color the presentation far more than a properly designed active circuit might.

The logical extension of this property is the proper use of balanced lines. If the balanced line standard (AES48) is observed and if the cables are operated with low impedances (which means the line stage driving the cable is actually a small power amp capable of driving 600 Ohms) then the cables have no audible differences/colorations between them. Of course, most 'high end audio' products I've seen do not support the requirements I just laid out, so there is some controversy about whether they are better. 

A corollary is that I would expect an amp with lots more circuitry will have more distortion than a simpler circuit

That depends on how much loop gain the amp has and if it has feedback!

Loop gain is the gain of the amp plus the amount of feedback, usually expressed in dB. So if the amp has 25 dB of gain and 20dB of feedback its overall loop gain (open loop) is 45dB. To get more loop gain the circuit is often more complex. Class D circuits (if self oscillating), due to how they make gain, can be a bit simpler. But even then, to reduce distortion further the pursuit of loop gain causes the class D circuit to be ever more complex. But the result is usually lower distortion. 

The simplest circuits, such as SETs, which usually operate without feedback, also have the most distortion, often at 10% at 'clipping'. They soft clip very well so that term is a bit of a moving target.

But there are also fairly simple circuits that are low distortion so I don't think its a good idea to generalize on this topic. 

Perhaps you have the question backwards:  a pure solitary note is a musical distortion.  No analog musical instruments produce one pure note.  The reason different instruments sound differently while playing the same note is due to the associated harmonics of that instrument, which some are calling “distortion”, when that “distortion,” when accurately reproduced, is the true sound of the musical instrument.  (Modern electronic so-called “music” might be the exception to this, depending on the engineering and circumstances,)

@drbond try tuning a piano or any string instrument with tuning forks, play the instrument for a score or two and then confirm the tuning with the same tuning forks again. Beating, oscillations, or warbling?

@devinplombier ”Yes, it is only available on Apple products”

A bit off topic, but l have never “warmed” to Apple products.

Apple expect their customers to pay more for the equivalent hardware than their rivals. For the £$ comparative reasoning alone, l have only ever used Apple products when they have been free hand-me-downs.